Gain-switched Laser Research Articles

Effect of repetition rate on gain-switched fiber laser output pulses

We investigate the influence of repetition rate on the operation and output of a gain-switched fiber laser. We derive an equation relating pump pulse duration to repetition rate, which provides insight into the parameters of the operating laser such as absorbed pump power and internal losses. The change in laser pulse duration with varying repetition rate is measured and found to be negligible. Finally, the derived theory is compared to experimental results obtained using a ytterbium-doped rod type fiber laser.

High-fidelity all-fiber amplification of a gain-switched laser diode.

We demonstrate a multistage erbium fiber amplifier seeded by a gain-switched laser diode operating at a wavelength of 1550 nm and a repetition rate of 1 MHz. The pulse energy is 0.5 μJ, and the pulse duration is 40 ps, resulting in a peak power of 11.4 kW. The three-stage all-fiber amplifier system is designed to avoid any spectral distortions induced by gain saturation or nonlinear effects and high levels of amplified spontaneous emission. The output pulses are close to transform limited with a Gaussian pulse envelope.

Robust random number generation using steady-state emission of gain-switched laser diodes

We demonstrate robust, high-speed random number generation using interference of the steady-state emission of guaranteed random phases, obtained through gain-switching a semiconductor laser diode. Steady-state emission tolerates large temporal pulse misalignments and therefore significantly improves the interference quality. Using an 8-bit digitizer followed by a finite-impulse-response unbiasing algorithm, we achieve random number generation rates of 8 and 20 Gb/s, for laser repetition rates of 1 and 2.5 GHz, respectively, with a ±20% tolerance in the interferometer differential delay. We also report a generation rate of 80 Gb/s using partially phase-correlated short pulses. In relation to the field of quantum key distribution, our results confirm the gain-switched laser diode as a suitable light source, capable of providing phase-randomized coherent pulses at a clock rate of up to 2.5 GHz.

Picosecond supercontinuum light source for stroboscopic white-light interferometry with freely adjustable pulse repetition rate.

We present a picosecond supercontinuum light source designed for stroboscopic white-light interferometry. This source offers a potential for high-resolution characterization of vibrational fields in electromechanical components with frequencies up to the GHz range. The light source concept combines a gain-switched laser diode, the output of which is amplified in a two-stage fiber amplifier, with supercontinuum generation in a microstructured optical fiber. Implemented in our white-light interferometer setup, optical pulses with optimized spectral properties and below 310 ps duration are used for stroboscopic illumination at freely adjustable repetition rates. The performance of the source is demonstrated by characterizing the surface vibration field of a square-plate silicon MEMS resonator at 3.37 MHz. A minimum detectable vibration amplitude of less than 100 pm is reached.

Spectral dynamics of picosecond gain-switched pulses from nitride-based vertical-cavity surface-emitting lasers

Short pulses generated from low-cost semiconductor lasers by a simple gain-switching technique have attracted enormous attention because of their potential usage in wide applications. Therein, reducing the durations of gain-switched pulses is a key technical point for promoting their applications. Therefore, understanding the dynamic characteristics of gain-switched pulses is highly desirable. Herein, we used streak camera to investigate the time- and spectral-resolved lasing characteristics of gain-switched pulses from optically pumped InGaN single-mode vertical-cavity surface-emitting lasers. We found that fast initial components with ultra-short durations far below our temporal resolution of 5.5 ps emerged on short-wavelength sides, while the entire pulses were down-chirped, resulting in the simultaneous broadening of the spectrum and pulse width. The measured chirp characteristics were quantitatively explained using a single-mode rate-equation model, combined with carrier-density-dependent gain and index models. The observed universal fast short-wavelength components can be useful in generating even shorter pulses from gain-switched semiconductor lasers.

27aAU-13 半導体レーザーの利得スイッチングにおける空間的な非一様性の影響(27aAU 量子細線・量子井戸・超格子・光応答,領域4(半導体,メゾスコピック系・局在))

27aAU-13 半導体レーザーの利得スイッチングにおける空間的な非一様性の影響(27aAU 量子細線・量子井戸・超格子・光応答,領域4(半導体,メゾスコピック系・局在))

7-ps optical pulse generation from a 1064-nm gain-switched laser diode and its application for two-photon microscopy

In this study, we investigated the picosecond optical pulse generation from a 1064-nm distributed feedback laser diode under strong gain switching. The spectrum of the generated optical pulses was manipulated in two different ways: (i) by extracting the short-wavelength components of the optical pulse spectrum and (ii) by compensating for spectral chirping in the extracted mid-spectral region. Both of these methods shortened the optical pulse duration to approximately 7 ps. These optical pulses were amplified to over 20-kW peak power for two-photon microscopy. We obtained clear two-photon images of neurons in a fixed brain slice of H-line mouse expressing enhanced yellow fluorescent protein. Furthermore, a successful experiment was also confirmed for in vivo deep region H-line mouse brain neuron imaging.

Gain switch laser based on micro-structured Yb-doped active fiber

In this paper a near infrared gain-switched fiber laser based on oscillator stage only design with high peak power is presented. Output pulses reached 2.3 kW of peak power and duration of less than 60 ns. The dependence of the laser pulse duration on operation parameters was measured and theoretically explained. As the setup is based on flexible micro structured single polarization fiber, the laser output exhibits high polarization extinction ratio. Due to the narrow output spectrum the setup is suitable for second harmonic generation.

Microsecond gain-switched master oscillator power amplifier with high pulse energy

An all-fibre master oscillator power amplifier (MOPA) emitting high-energy pulses at is presented. The seed laser is a microsecond gain-switched thulium-doped fibre laser (TDFL) pumped with a commercial pulsed fibre laser. The TDFL operates at a repetition rate in the range of 10 to . The two-stage thulium-doped fibre amplifier is built to scale the energy of the pulses generated by the seed laser. The maximum output pulse energy higher than at is achieved which is comparable with the theoretical maximum extractable pulse energy. The slope efficiency of the second stage amplifier with respect to the pump power is 30.4 % at . The wavelength of the output pulse laser is centred near at a spectral width of after amplification. Neither nonlinear effects nor significant amplified spontaneous emission (ASE) is observed in the amplification experiments.

Ultra-fast quantum randomness generation by accelerated phase diffusion in a pulsed laser diode

We demonstrate a high bit-rate quantum random number generator by interferometric detection of phase diffusion in a gain-switched DFB laser diode. Gain switching at few-GHz frequencies produces a train of bright pulses with nearly equal amplitudes and random phases. An unbalanced Mach-Zehnder interferometer is used to interfere subsequent pulses and thereby generate strong random-amplitude pulses, which are detected and digitized to produce a high-rate random bit string. Using established models of semiconductor laser field dynamics, we predict a regime of high visibility interference and nearly complete vacuum-fluctuation-induced phase diffusion between pulses. These are confirmed by measurement of pulse power statistics at the output of the interferometer. Using a 5.825 GHz excitation rate and 14-bit digitization, we observe 43 Gbps quantum randomness generation.

The all-fiber cladding-pumped Yb-doped gain-switched laser

Gain-switching is an alternative pulsing technique of fiber lasers, which is power scalable and has a low complexity. From a linear stability analysis of rate equations the relaxation oscillation period is derived and from it, the pulse duration is defined. Good agreement between the measured pulse duration and the theoretical prediction is found over a wide range of parameters. In particular we investigate the influence of an often present length of passive fiber in the cavity and show that it introduces a finite minimum in the achievable pulse duration. This minimum pulse duration is shown to occur at longer active fibers length with increased passive length of fiber in the cavity. The peak power is observed to depend linearly on the absorbed pump power and be independent of the passive fiber length. Given these conclusions, the pulse energy, duration, and peak power can be estimated with good precision.

Single stage Yb-doped fiber laser based on gain switching with short pulse duration

A simple solution for producing nanosecond laser pulses can be obtained using gain-switched fiber lasers. In this paper, we present an optimized single stage gain-switched ytterbium-doped fiber laser. Three fiber lengths were tested to show the impact of length on the laser output pulse. A pulse as short as 28 ns at 1.4 kW peak power and a maximum peak power of nearly 2 kW at 41 ns pulse duration was achieved. The laser possess a linear polarized output, very good beam quality of M2 < 1.1, and a spectral bandwidth of 0.11 nm.

High Power Supercontinuum Generation in Fluoride Fibers Pumped by 2 $\mu{\rm m}$ Pulses

High power supercontinuum generation in fluoride step-index optical fibers pumped with 2- μm pulses delivered by a gain-switched thulium-doped fiber laser and amplifier system is reported. An output average power of 1.25 W in the ~ 1.8-4.15 μm spectral band and 1.82 W in the ~ 1.8-3.65 μm band was achieved for two fluorozirconate fibers of different parameters. The first demonstration of Watt-level supercontinuum generation in a fluoroindate fiber is also demonstrated. The achieved output SC powers are the highest ones ever reported for fluoride fibers pumped with 2- μm gain-switched optical pulses.

Temporal characteristics of in-band-pumped gain-switched thulium-doped fiber lasers

We numerically investigated temporal characteristics of in-band-pumped gain-switched thulim-doped fiber lasers (GSTDFL). Our theoretical model was based on rate equations and optical power propagation equations including both temporal and spatial variations. This model was validated by comparing the numerical results with results from previous experimental work. Due to the lack of a clear definition of the numerical threshold of gain-switched fiber lasers in the literature, we first defined it in our calculations as the status obtained when the variation of the widths of the generated pulses became nonlinear. Then, we used the model to explore the temporal characteristics of such lasers at and above the laser threshold. We found that the threshold pump energy would increase when the pump pulse width was longer than 500&#xA0;ns and the pulse widths of the laser pulses at the threshold were kept the same at different pump widths. Above the threshold, by calculating the temporal shapes of the laser pulses under different pump conditions, we studied the origin of the trailing spike phenomenon in the in-band-pumped GSTDFLs and determined the parameter space of the pump conditions that could generate the pulses having a Gaussian-like temporal shape and free from the trailing spikes. We further investigated the variation of the laser pulse widths at different laser configurations and discussed their implications. We believe these numerical results can be referenced qualitatively by the design and optimization of the in-band-pumped GSTDFLs and other in-band-pumped gain-switched fiber lasers, such as those with Yb3+ and Ho3+ dopants.

Performance improvement by a saturable absorber in gain-switched asymmetric-waveguide laser diodes

A simplest saturable absorber, in the form of an unpumped section, is introduced into a Fabry-Perot semiconductor laser with a strongly asymmetric broadened waveguide structure incorporating a relatively thick (80 nm) active layer. This allows for suppression of trailing oscillations and a decrease in the optical pulse width compared to the uniformly biased structure. Single optical pulses of ~80 ps full width at half maximum (FWHM) and ~35 W peak power (~3 nJ pulse energy, E(opt)), practically without trailing edge oscillations, were experimentally achieved under room temperature conditions by absorber-assisted gain-switching, using pumping current pulses of ~1.3 ns FWHM and ~17 A amplitude. The laser emission has a narrow (13 degrees FWHM in the transverse direction) far field.

Gain-switched multicarrier transmitter in a long-reach UDWDM PON with a digital coherent receiver

The authors report on the downlink performance of a 10 Gb/s long-reach and ultra-dense wavelength-division multiplexed passive optical network, based on a multicarrier transmitter realized by using an externally injected gain-switched distributed-feedback laser diode. Each of the comb channels, spaced by 10 GHz, is modulated with a 3 Gbaud dual polarization quadrature phase shift keying signal that included a 20% overhead for forward error correction. Frequency selectivity and enhanced receiver sensitivity is achieved by employing a digital coherent receiver to receive the signal. Experimental results achieved in a back-to-back and 100 km transmission scenarios show an excellent worst case receiver sensitivity of -44 dBm.

Photonic generation of ultrawideband signals based on a gain-switched semiconductor laser with optical feedback

A simple photonic approach to generate ultrawideband (UWB) pulse signals utilizing a gain-switched semiconductor laser with optical feedback is proposed and demonstrated. The RF spectrum of the generated chaotic UWB signals has a -10 dB bandwidth of 9 GHz and central frequency of 6.6 GHz (fractional bandwidth of 155%), which is consistent with the Federal Communications Commission indoor mask. The central frequency and -10 dB bandwidth can be tuned by adjusting the bias current and feedback strength of the semiconductor laser. After transmission through a 30 km single-mode fiber, the spectrum shape of the chaotic UWB signals is almost unaffected by the chromatic dispersion of the fiber.

Stable passively Q-switched and gain-switched Yb-doped all-fiber laser based on a dual-cavity with fiber Bragg gratings

We demonstrate a stable passively Q-switched and gain-switched Yb-doped all-fiber laser cladding-pumped by a continuous fiber-coupled 976 nm laser diode. By use of an all-fiber dual-cavity, the efficient elements of the laser mainly include the fiber Bragg gratings and rare-earth doped fiber, allowing the oscillator to be integrated in a compact size with reliable and stable output. In this scheme, an efficient laser output with 45 ns pulse width, 62 μJ pulse energy, and 1.4 kW peak power operating at 1081 nm was obtained. To the best of our knowledge, this is the minimum pulse width in this similar kind of all-fiber configuration at present. Sequential nanosecond pulses were obtained at the repetition rate of several to tens of kHz with the variation of the diode pumping power. Effects of laser parameters such as pump power, cavity length, external-cavity wavelength, and FBG reflectivity on laser performance were also presented and discussed.

Temporal characteristics of gain-switched thulium-doped fiber laser near threshold

The temporal output characteristics of an all-fiber gain-switched thulium-doped fiber laser near threshold are investigated experimentally and numerically. The pulse repetition rate and pulse energy of the 1550&#xA0;nm pump source are found to be two important parameters that affect the laser performance. Adjusting these two parameters, three different output working states, namely no lasing state, unstable state, and steady state, are observed experimentally. Particularly, in the unstable state relatively stable output pulse trains with repetition rate trisecting (PRR 1/3) and halving (PRR 1/2) with respect to the pump source are found. The experimental results together with corresponding numerical simulation afford a deep insight into the related lasing mechanisms.

Side mode suppression and dispersion compensation analysis of a 60 GHz radio-over-fibre system based on a gain switched laser

The research and technical community have designated a band of 7GHz between 57 and 64GHz for short-range wireless communications. This paper utilizes a simple and cost effective technique for generating a 60GHz millimeter-wave (mm-wave) signal using an optical comb source based on a gain-switched laser (GSL). This research investigates the effects unwanted comb lines have on the overall system performance with 2.5Gb/s data transmission. To do this, a programmable optical filter is used to suppress the unwanted comb lines to varying levels. Bit-error rate (BER) measurements were carried out against received optical power to demonstrate the detrimental effects the unwanted comb lines have on the modulated mm-wave signal when not sufficiently suppressed. As chromatic dispersion is a limiting factor to the system's transmission distance, this work also investigates pre-compensation for dispersion utilizing the programmable group delay capabilities of the programmable optical filter, demonstrating the ability to extend the transmission distance by 12km. All experimental results obtained are reinforced through simulation.