Tunable Pulse Width Research Articles

2166 nm all-fiber short-pulsed Raman laser based on germania-core fiber.

We report a compact 2166 nm germania-fiber short-pulsed Raman laser based on the cavity matching scheme. The all-fiber Raman cavity is formed by a pair of 2166 nm fiber Bragg gratings. High-power noise-like pulses from a 1981 nm fiber laser are used to pump a 22 m germania-core fiber for providing Raman gain at ∼2166 nm, and readily realizes the Raman-cavity synchronization with high mismatching tolerance. Stable Raman pulses at 2166 nm are therefore generated with the tunable pulse width of 0.9-4.4 ns and the large pulse energy up to 12.15 nJ. This is, to the best of our knowledge, the first demonstration of all-fiber short-pulsed Raman laser in the mid-infrared region.

Pulse delay-controlled passive Q-switched laser with tunable pulse width

We present a method for obtaining a passive Q-switched Nd : YAG / Cr4 + : YAG laser with tunable pulse width. The output pulse width is tuned based on the delay time between the pumping pulse and trigger pulse. An auxiliary laser with a narrow pulse is used to trigger the laser pulse generation at different delay times during the pumping pulse duration. It is found that the output pulse width can be continuously adjusted from 1415 to 562 ps with an output peak power ranging from 44 to 240 kW at a fixed pumping power. The beam quality factor M2 is kept <1.5 with changing of the pulse width.

Laser-Diode Pumped Passive Q-Switched Laser with Quick Tunable Pulse-Width Capability

A laser-diode pumped passive Q-switched Nd:YAG/Cr4+:YAG laser with quick tunable pulse-width capability is studied and experimentally demonstrated. By focusing the auxiliary controlling laser beam onto a Cr4+:YAG crystal and thereby changing the initial transmission of the Cr4+:YAG crystal, laser pulses with different pulse widths can be achieved. The obtained laser pulse width is tunable in the range of 550–1492 ps with a fixed cavity length of 7 mm. The laser can be used to achieve different pulse widths between two pulses in the range of sub-nanosecond to a few tens of nanoseconds.

Mode-locked thulium-fluoride fibre laser with an adjustable pulse width using a nonlinear optical loop mirror

A mode-locked thulium-fluoride fibre laser with a tunable pulse width using a nonlinear optical loop mirror (NOLM) for operation in the S-band region is proposed and demonstrated. Stable dissipative soliton resonance (DSR) mode-locked pulses are obtained at a threshold pump power of 186.8 mW and maintain stability until a maximum pump power of 249.7 mW. Pulse width modulation of between 70.8 ns and 120.3 ns is obtained over the operating pump power range, with a fundamental frequency at 97.4 kHz. A maximum output power of 0.99 mW and a pulse energy of 10.21 nJ are obtained. The output pulse has a signal-to-noise ratio of 69 dB at the maximum pump power, indicating a highly stable output. The variable pulse width will find significant applications as modulated light sources for various optical and optoelectronic systems.

Highly stable adjustable gain-switched semiconductor pulsed seed source

To intelligently control the parameters of a semiconductor pulsed seed source, a novel scheme is presented involving commercial electronic chips as switching devices to drive a semiconductor laser (SL). By combining theoretical analysis and rate-equation theory, we construct a mathematical model of the SL. Further, a pulsed laser module with real-time tunable optical pulse parameters is designed and implemented. The tunable pulse width ranges from 90 ps (PS) to continuous wave, and the tunable pulse repetition rate is 0–10 MHz. the SL temperature drift is less than 0.07 °C. The experimental results verify the feasibility of the design scheme, and the theoretical analysis and simulation results guide the generation of high-quality optical pulses.

Tunable all-optical actively mode-locked fiber laser at 2 µm based on tellurite photonic crystal fiber

Although thulium-doped fiber lasers operating at 2 µm have been intensively investigated, lasers with high repetition rate (up to several tens of gigahertz) and tunable output characteristics are difficult to achieve at this band. In this paper, an all-optical actively mode-locked fiber laser is proposed for this purpose. By employing a section of specially-designed highly nonlinear group-velocity-matched tellurite photonic crystal fiber as the mode-locker, the proposed thulium-doped fiber laser can easily produce stable pulses with tunable pulse width and repetition rate up to 40 GHz. Moreover, through investigating the pulse dynamics in the cavity, a transfer-limited soliton pulse is successfully achieved. The proposed laser can find application in future optical fiber communication systems at 2 µm.

Diverse mode of operation of an all-normal-dispersion mode-locked fiber laser employing two nonlinear loop mirrors.

In this paper, we propose an all-normal-dispersion ytterbium-fiber laser with a novel ring cavity architecture having two nonlinear amplifying loop mirrors (NALM) as saturable absorbers, capable of delivering distinctly different pulses with adjustable features. By optimizing the loop lengths of the individual NALMs, the cavity can be operated to deliver Q-switched mode-locked (Q-ML) pulse bunches with adjustable repetition rates, mode-locked pulses in dissipative soliton resonance (DSR) regime or noise-like pulse (NLP) regime with tunable pulse width. The DSR pulses exhibit characteristic narrowband spectrum, while the NLPs exhibit large broadband spectrum. The operation regime of the laser can be controlled by adjusting the amplifier pump powers and the polarization controllers. To the best of the authors' knowledge, this is the first demonstration of a single mode-locked cavity where narrowband DSR pulses and broadband NLPs alongside Q-ML pulse bunches can be selectively generated by employing two NALMs.

All-Optical Actively Mode-Locked Fiber Laser at 2-μm Based on Interband Modulation

In this paper, we have achieved a high repetition rate and tunable pulse generation near ∼2 μm through an all-optical actively mode-locked fiber laser. The performances of the fiber laser have been systematically investigated through numerical simulations. The thulium-doped fiber laser cavity is modulated by the optical injection at 1.55 μ m via a nonlinear optical loop mirror in which a specially-designed group-velocity-matching highly nonlinear fiber is incorporated. According to the simulation results, up to 40-GHz pulse train with picosecond tunable pulse width and peak power can be produced. Moreover, the repetition rate can be further enlarged with the help of higher gain. The impacts of gain coefficient, cavity length, and repetition rate on the output are studied in details. The proposed mode-locked fiber laser can be potentially used for future 2- μ m fiber communication.

Experimental optimization of dissipative soliton resonance square pulses in all anomalous passively mode-locked fiber laser

We investigate experimentally the operational boundaries of dissipative soliton resonance in a double-clad Er:Yb co-doped dual amplifier passively mode-locked figure-of-eight fiber laser. While mode-locking with a nonlinear amplifying loop mirror, we present an exhaustive series of experiments in order to optimize the pulse energy and pulse width tunability. In addition to the pumping power of the amplifiers, several key experimental parameters have been identified such as the net cavity dispersion, the coupling ratio between the two loops of the cavity and the exact position of the long fiber coils. Under optimized conditions, the laser delivers square pulses with an energy varying between 8.5 and 10.1 μJ while the pulse width ranges from 84–416 ns.

Broadband tunable integrated CMOS pulser with 80-ps minimum pulse width for gain-switched semiconductor lasers

High power pulsed lasers with tunable pulse widths are highly favored in many applications. When combined with power amplification, gain-switched semiconductor lasers driven by broadband tunable electric pulsers can meet such requirements. For this reason, we designed and produced a low-cost integrated CMOS pulse generator with a minimum pulse width of 80 ps and a wide tuning range of up to 270 ns using a 40-nm microelectronic process technique. We used this pulser to drive a 1.3-µm semiconductor laser diode directly, and thereafter investigated the gain-switching properties of the laser system. The optical pulses consist of a spike followed by a steady state region. Tuning the width of the electrical pulse down to approximately 1.5 ns produces optical pulses consisting only of the spike, which has a minimum pulse-width of 100 ps. Moreover, the duration of the steady state can be tuned continuously by tuning the electrical pulse width, with a peak power of approximately 5 mW. The output voltage of the electric pulser has a tuning range of 0.8–1.5 V that can be used to directly drive semiconductor laser diodes with wavelengths in the near-infrared spectrum, which are suitable for power amplification with rare-earth doped fiber amplifiers.

Quantum dot cadmium selenide as a saturable absorber for Q-switched and mode-locked double-clad ytterbium-doped fiber lasers

This paper demonstrates the integration of quantum dot (QD) cadmium selenide (CdSe) nanoparticles, which is embedded into polymethyl methacrylate (PMMA) film into an ytterbium-doped fiber laser (YDFL) cavity to produce Q-switched and mode-locked fiber lasers. The QD CdSe based film functions as a saturable absorber (SA). For Q-switching operation, stable pulse is generated within 970–1200mW pump power, with tunable repetition rate and pulse width of 24.5–40.5kHz and 6.8–3.7µs, respectively. Maximum pulse energy and peak power are obtained about 1.1µJ and 0.28W, respectively. As we tune the polarization state of the laser cavity and use a single QD CdSe film, the mode-locking operation could also be generated within 310–468mW pump power with repetition rate of 14.5MHz and pulse width of 3.5ps. Maximum pulse energy and peak power are obtained about 2nJ and 0.11W, respectively. These results may contribute to continuous research work on laser pulse generation, providing new opportunities of CdSe material in photonics applications.

Ultrafast Broadband Saturable Absorption in Sb2Se3 Nanowires

Manipulating optical nonlinearity is a very fascinating prospective for both fundamental and technological purpose; offers a potential platform to explore the saturable absorption (SA) to the nanoscale limit. In this manuscript, we have focused on the tunable wavelength and pulse width dependent ultrafast broadband SA of Sb2Se3 nanowires in the resonant and the above bandgap excitations of 120 fs and 7 ns pulses. In this context, we numerically simulated the SA of Sb2Se3 nanowire with a two-level model. The solutions of the coupled rate equations for a Gaussian shaped pulse exactly reproduced the experimental results in both temporal and spatial domains. Our model provides a precise spacio-temporal analysis of carrier populations of each photo-excited state in the system. Moreover, excited state decay time (1.5 ns) and the ratio of excited state absorption cross section (σes) to the ground state absorption cross section (σgs) is less than unity for 1-D Sb2Se3 nanowires. Hence, Sb2Se3 nanowires can be employed as passive mode-lockers and essential photodetectors for ultrashort pulsed laser and can be tuned by wavelength and pulse width. These observations advise a remarkable nonlinear optical response which is expected to be comparable or even better than the graphene and 2D TMDC’s (MX2) like MoSe2, MoS2, WSe2, WS2 etc.

Unintentional Polarization Dependent Pulsewidth of Graphene Mode-Locked Er-Doped Fiber Lasers

The unique polarization dependent pulsation phenomenon in a graphene mode-locked erbium-doped fiber laser (ML-EDFL) is explored by the interaction between intracavity weak nonlinear polarization rotation and strong saturable absorption. With unintentional polarization dependent loss (PDL) induced by twisting single-mode fiber (SMF) in an intracavity polarization controller (PC), the unique pulsewidth tunability in a polarizer-free graphene ML-EDFL can be observed. Numerical and experimental procedures are established to combine the hybrid effects and to realize the possible detuning range of intracavity polarization angle for polarization dependent mode-locking dynamics. Twisting SMF induced unintentional PDL in the polarizer-free EDFL contributes a tiny self-amplitude modulation (SAM) coefficient of ${\gamma _{\rm NPR}} = 5.36 \times {10^{ - 4}}\sin 2{\theta _P}$ . The mode-locking stability still relies on the graphene saturable absorption to optimize the SAM up to ${\gamma _{\rm total}} = 1.46 \times {10^{ - 3}}$ and shorten the EDFL pulsewidth to 330 fs. These clearly elucidate the enigma on polarization dependency of most polarizer-free ML-EDFLs with built-in PC. Even an unintentional weak polarization dependency in the EDFL without any polarizer can induce a residual PDL to introduce a polarization angle-dependent SAM coefficient, which declares the importance on controlling intracavity polarization for stabilizing and optimizing the graphene ML-EDFL with unintentional nonlinear polarization rotation induced PDL.

Pulsed Pumping for Pulsewidth Tunable Nanosecond Ytterbium-Doped Fiber Laser

High efficiency, good robustness, and low amplified spontaneous emission are among the key factors of the high power ytterbium-doped pulsed fiber laser. In this letter, we propose an optimized pulsed pumping design for the nanosecond ytterbium-doped fiber laser with tunable pulse repetition rate and pulsewidth. For the pulsed pumping, the pumping power is kept at the maximal reachable power level and the pumping duration is modulated according to the pulsewidth. At each pulsewidth, the pumping duration is fixed for every seeding pulse. The performance of this laser at 130- and 20-ns seeding pulsewidth is experimentally analyzed. Simulation results are provided with the same configuration of the experiments. The results show that better performance of the laser is achieved. This technique is meaningful and strongly recommended for the design of high power nanosecond ytterbium-doped fiber laser.

Flexible pulses from carbon nanotubes mode-locked fiber laser**Project supported by the National Natural Science Foundation of China (Grant No. 61575137) and the Program on Social Development by Department of Science and Technology of Shanxi Province, China (Grant No. 20140313023-3).

We demonstrate a flexible erbium-doped pulsed fiber laser which achieves the wavelength and pulse width tuning by adjusting an intracavity filter. The intracavity filter is flexible to achieve any of the different wavelengths and bandwidths in the tuning range. The wavelength and width of pulse can be tuned in a range of ∼ 20 nm and from ∼ 0.8 ps to 87 ps, respectively. The flexible pulsed fiber laser can be accurately controlled, which is insensitive to environmental disturbance.

Multi-function Mach-Zehnder modulator for pulse shaping and generation.

We present a multi-function electronic-photonic integrated circuit (EPIC) design which exploits a new operation mode of a Mach-Zehnder modulator (MZM). Different from the conventional design, the two arms of the modulator are driven by time-shifted signals of tunable amplitude. We study its operation in the linear and quadratic regions where the MZM is biased at π/2 and π initial phase difference, respectively. In the linear region, the modulator sums the waveforms of the driving signals in the two arms, which can be used to add pre-emphasis function to the modulator, and hence it obviates an electrical pre-emphasis driver. Furthermore, when operating in the quadratic region, the modulator can produce optical pulses with tunable pulse width at double clock rate. Prototype circuits are designed first using a suit of device, electromagnetic simulators to build compact models, and then importing into a photonic circuit simulator for complete circuit performance evaluation.

Reconfigurable symmetric pulses generation using on-chip cascaded optical differentiators.

We report a type of programmable pulse shaping method based on cascaded frequency-detuned optical differentiators. By properly adjusting the central wavelength of each differentiator, a large variety of symmetric pulses can be generated from a transform-limited Gaussian-like pulse. We numerically and experimentally demonstrate the generation of flat-top, parabolic and triangular pulses with tunable pulse widths from a 20-ps Gaussian-like pulse, using no more than three cascaded differentiators. It can be found that as more differentiators are used, higher synthesized accuracy and larger tuning range of pulse widths can be obtained in general. Additionally, in our experiment, we design and fabricate thermally tunable delay interferometers on the silicon-on-insulator (SOI) platform to work as optical differentiators, which can help us realize the shaping system with small footprint (943μm × 395μm) and high stability.

Preclinical photoacoustic models: application for ultrasensitive single cell malaria diagnosis in large vein and artery.

In vivo photoacoustic flow cytometry (PAFC) has demonstrated potential for early diagnosis of deadly diseases through detection of rare circulating tumor cells, pathogens, and clots in nearly the entire blood volume. Before clinical application, this promising diagnostic platform requires verification and optimization using adequate preclinical models. We show here that this can be addressed by examination of large mouse blood vessels which are similar in size, depth and flow velocity to human vessels used in PAFC. Using this model, we verified the capability of PAFC for ultrasensitive, noninvasive, label-free, rapid malaria diagnosis. The time-resolved detection of delayed PA signals from deep vessels provided complete elimination of background from strongly pigmented skin. We discovered that PAFC's sensitivity is higher during examination of infected cells in arteries compared to veins at similar flow rate. Our advanced PAFC platform integrating a 1060 nm laser with tunable pulse rate and width, a wearable probe with a focused transducer, and linear and nonlinear nanobubble-amplified signal processing demonstrated detection of parasitemia at the unprecedented level of 0.00000001% within 20 seconds and the potential to further improve the sensitivity 100-fold in humans, that is approximately 106 times better than in existing malaria tests.

Tunable and Switchable Narrow Bandwidth Semiconductor-Saturable Absorber Mirror Mode-Locked Yb-Doped Fiber Laser Delivering Different Pulse Widths**Supported by the National High Technology Research and Development Program of China under Grant No 2014AA041901, NSAF Foundation of the National Natural Science Foundation of China under Grant No U1330134, the Opening Project of Shanghai Key Laboratory of All Solid-State Laser and Applied Techniques under

The wavelength-tunable and switchable narrow bandwidth mode-locking operation is demonstrated in an all fiber laser based on semiconductor-saturable absorber mirror (SESAM). Two narrow-band fiber Bragg gratings centered at 1029.9 nm and 1032 nm respectively with a polarization controller inserted between them are used to realize the wavelength switchable between 1029.9 nm and 1032 nm. The laser delivers different pulse widths of 7.5 ps for 1030 nm and 20 ps for 1032 nm. The maximum output power for both could reach ~6.5 mW at single pulse operation. The output wavelength could be tuned to about 0.9 nm intervals ranging from 1030.2 nm to 1031.1 nm and from 1032.15 nm to 1033.7 nm with the temperature change of the fiber Bragg grating, respectively.

All-optical pulsewidth-tunable wavelength conversion of return-to-zero differential phase-shift keying signal

We demonstrate the wavelength conversion of return-to-zero differential phase-shift keying (RZ-DPSK) signal with the pulsewidth-tunable operation using a Raman amplifier (RA)-based ultrashort pulse compressor and a highly nonlinear fiber (HNLF)-based four-wave mixing (FWM) switch. The wavelength of the input RZ-DPSK signal is converted to new wavelengths owing to the FWM effect in HNLF. On the other hand, the pulsewidth of the wavelength-converted RZ-DPSK signal is tuned from 10 to 3 ps by using the ultrashort pulse source based on RA-based adiabatic soliton pulse compression. The bit error rate characteristics of the input RZ-DPSK and the wavelength-converted RZ-DPSK signals with different pulsewidths are measured and error-free operations are achieved in all cases. The optical spectra, autocorrelation traces, and clear and opening eye diagrams show high conversion performance with wide-range pulsewidth tunability.