Gain-switched Laser Research Articles

High-power deep-ultraviolet light generation at 266 nm from frequency quadrupling of a picosecond pulsed 1064 nm laser with a Nd:YVO4 amplifier pumped by a 914 nm laser diode.

We report the generation of picosecond pulsed light at a 266 nm wavelength with an average power of 53 W. We developed a picosecond pulsed 1064 nm laser source with an average power of 261 W, a repetition rate of 1 MHz, and a pulse duration of 14 ps, using a gain-switched DFB laser diode as a seed laser and a 914 nm laser-diode-pumped Nd-doped YVO4 power amplifier. We achieved stable generation of 266 nm light with an average power of 53 W from frequency quadrupling using an LBO and a CLBO crystals. The amplified power of 261 W and the 266 nm average power of 53 W from the 914 nm pumped Nd:YVO4 amplifier are the highest ever reported, to the best of our knowledge.

Short pulses from a gain-switched quantum cascade laser

The picosecond carrier dynamics observed in quantum cascade lasers (QCLs) poses a fundamental obstacle for the formation of intracavity pulses. On the other hand, the ultrafast gain response makes the QCL ideally suited for high frequency modulation of its pump current. In this work, we leverage this property and use short electrical excitations to generate 33 ps optical pulses with up to Watt level peak power. We ensure lasing on a single resonator mode via optical injection seeding. The resulting pulses are characterized in both time and frequency domains using an optical sampling method in combination with a spectrometer. We interpret the obtained results using laser rate equations. The results presented in this work open new pathways for the generation of highly tunable, high power mid-infrared pulses from a monolithic source.

Multi-band triangular frequency modulation signal generation based on gain-switched laser

A multi-band triangular frequency modulation signal generation method is proposed and verified by simulation and experiment. This is a simple photonics method based on distributed feedback gain switching states of semiconductor lasers without the need for complex external modulators. The carrier frequency and bandwidth of the generated triangular frequency modulation signal are N times that of the fundamental signal (N is the number of comb lines). By tuning the bandwidth and duration of the fundamental signal, the product of the number of frequency bands and the time bandwidth (TBWP) of the generated signal can be easily adjusted. The autocorrelation results of the generated waveform are also given. The triangular frequency modulation signals of six bands from S-band to K-band are realized, and have different chirp rates. The maximum TBWP reaches 24000, and the corresponding range resolution reaches 6.3 cm.

High-density and broad band optical frequency combs generated by pseudo-random phase modulation of optically injected gain-switched semiconductor lasers

We have demonstrated optical frequency combs (OFCs) featuring low and continuously tunable line spacing in the kilohertz range. The OFCs are generated by pulsed gain-switching of optically injected commercial semiconductor lasers emitting at around 1.55 μm. After generation, the OFCs are efficiently densified by pseudo-random-binary-sequence phase modulation. The case of an OFC generated by gain-switching at a repetition rate of 1 GHz has been numerically and experimentally analyzed and the conditions for optimal densification have been found. In this case, the comb has been densified by a factor of 2047 and exhibits a 90 GHz flat and broad spectrum consisting of more than 184,000 spectral lines, separated 488 kHz. Although this densification factor is not the largest that can be achieved with the method, it is more than 100 times larger than previously reported factors for gain-switching OFCs. These results open prospects for the use of potentially integrable gain-switching OFCs in applications requiring sub-megahertz line spacing.

Simple multi-band linearly frequency-modulated signal generation with a multiplying bandwidth based on a gain-switching laser.

Multi-band linearly frequency-modulated (LFM) signal generation with a multiplying bandwidth is proposed and experimentally demonstrated. It is a simple photonics method based on the gain-switching state in a distributed feedback semiconductor laser without a complex external modulator and high-speed electrical amplifiers. With N comb lines, the carrier frequency and bandwidth of generated LFM signals are N times those of the reference signal. (N is the number of comb lines.) The number of bands and time-bandwidth products (TBWPs) of the generated signals could be easily adjusted by tuning the reference signal from an arbitrary waveform generator. Three-band LFM signals with carrier frequencies ranging from the X-band to K-band are given as an example, and the TBWP up to 20000. The results of auto-correlations of the generated waveforms are also given.

Dysprosium gain-switched fiber laser at 3.24 µm pumped at 1064 nm

We report a dysprosium-doped gain switched fluoride fiber laser pumped at 1064 nm and emitting at 3244 nm. Pulses with energies up to 1 µJ were generated on a wide range of repetition rates between 100 kHz and 200 kHz, the highest reported for a pulsed dysprosium-doped fiber laser, producing an average output power reaching 207 mW. Stable 8.9 µJ pulses were also generated near 10 kHz, which is the highest pulse energy reported to date from a 1.1 µm band pumped dysprosium-doped fiber laser. This shows the 1.1 µm band as a valuable alternative for the pumping of dysprosium-doped fiber lasers emitting far from the emission peak and is thus of potential interest for future field applications.

Small-form-factor Gaussian-modulated coherent-state transmitter for CV-QKD using a gain-switched DFB laser.

We report a directly modulated distributed feedback laser operating in gain-switching mode for preparing the coherent states required for the Gaussian-modulated coherent-state (GMCS) continuous-variable quantum key distribution (CV-QKD) protocol. The proposed single-component quantum transmitter design eliminates the need for external modulators, decreasing the complexity of GMCS CV-QKD systems. The experimental results demonstrate a potential asymptotic secret key rate value of 2.63 Mbps over an 11-km fiber link, making the directly modulated GMCS transmitter particularly suitable for metropolitan optical networks where compactness, robustness, and low cost are key desirable features.

Phase randomness in a semiconductor laser: Issue of quantum random-number generation

Gain-switched lasers are in demand in numerous quantum applications, particularly, in systems of quantum key distribution and in various optical quantum random number generators. The reason for this popularity is natural phase randomization between gain-switched laser pulses. The idea of such randomization has become so familiar that most authors use it without regard to the features of the laser operation mode they use. However, at high repetition rates of laser pulses or when pulses are generated at a bias current close to the threshold, the phase randomization condition may be violated. This paper describes theoretical and experimental methods for estimating the degree of phase randomization in a gain-switched laser. We consider in detail different situations of laser pulse interference and show that the interference signal remains quantum in nature even in the presence of classical phase drift in the interferometer provided that the phase diffusion in a laser is efficient enough. Moreover, we formulate the relationship between the previously introduced quantum reduction factor and the leftover hash lemma. Using this relationship, we develop a method to estimate the quantum noise contribution to the interference signal in the presence of phase correlations. Finally, we introduce a simple experimental method based on the analysis of statistical interference fringes, providing more detailed information about the probabilistic properties of laser pulse interference.

Dynamics of 2nd quantized state laser oscillation in gain-switched quantum-well semiconductor lasers

The dynamics of second-quantized-state laser oscillation were investigated for semiconductor laser diodes with quantum-well structures inside. We found that the second-quantized state often dominates laser oscillation instead of the first-quantized state under intensive pulse excitation, while the DC bias superposition tends to suppress the second-quantized-state oscillation. The operation characteristics were studied in detail through experimental studies and numerical calculations.

Random-modulated pulse lidar using a gain-switched semiconductor laser with a delayed self-homodyne interferometer.

We propose the generation of random-modulated pulses using a gain-switched semiconductor laser with a delayed self-homodyne interferometer (DSHI) for lidar applications. By emitting non-repetitive random-modulated pulses, ambiguity in ranging and interference in detection can be mitigated. When gain-switched, the wavelength of the laser fluctuates abruptly at the beginning of the pulse and then drops until it stabilizes toward its continuous-wave (CW) state. By beating the two pulses with instantaneous frequency detuning from the DSHI, pulses consisting of random and down-chirped modulations can be generated without any complex code generation and modulation. In this study, we investigate the waveforms and spectra of the random-modulated pulses generated under various homodyne delay lengths, switching currents, and pulsewidths. We characterize their signal-to-noise ratio (SNR), precision, and cross-correlation between consecutive pulses to evaluate their performance in lidar applications. For a good SNR of over 12 dB, the generated pulses have an optimal precision of approximately 1 mm in ranging, which is substantially better than the chaos-modulated pulses generated based on laser feedback dynamics. By establishing a random-modulated pulse lidar based on the proposed gain-switched homodyne scheme, we successfully demonstrate 3D imaging and profiling with good precision.

A Field Trial of Multi-Homodyne Coherent Detection over Multi-Core Fiber for Encryption and Steganography

The concept of multi-homodyne coherent detection is used for stealth and secured optical communications, demonstrating a transmission rate of 20 Gbps under negative (in dB) optical signal-to-noise ratio (OSNR), in a deployed multi-core fiber (MCF) network. A 10 GBd QPSK signal is assigned with a bandwidth of multi-hundreds GHz, optically encoded with spectral phase mask, and transmitted below ASE noise in the MCF channel. To enable such transmission scheme, a semiconductor based gain-switched laser (GSL) frequency comb, used as both the signal carrier and local oscillator (LO), are then mixed in intradyne coherent receiver. The ultra-short time-domain overlapping between the pulsed signal and the pulsed LO allows the authorized user to collect the information, while discriminating most of the noise. Consequently, the electrical SNR is dramatically improved compared to the optical SNR, resulting in optical processing gain. Nevertheless, ultra-short pulses mixing is rather susceptible to a chromatic dispersion (CD), which imposes a major challenge in this spread-spectrum transmission technique. In this contribution, the effect of CD in multi-homodyne coherent detection is analyzed in an analytic model and in experiments. The model suggests a modified CD transfer function, comprised of the optical sampling of a conventional CD transfer function. In addition, the SNR penalty associated with the CD effect is quantified, as well as its dependency in propagation length and overall GSL's bandwidth. To circumvent the devastating effect of the CD, multi-core fibers are proposed. Two different weakly-coupled cores of a homogeneous MCF allow the signal and the LO to co-propagate through a very similar dispersive channel. A field trial in a deployed 6.29 km MCF network has validated an encrypted and stealthy 20 Gbps coherent transmission, at -2 dB OSNR.

L-band dual-comb based on gain-switched lasers for CO2 monitoring

This paper reports the development of a dual-comb spectroscopy system based on gain-switched single-mode laser diodes emitting at 1572 nm. The switching is performed by a train of short electrical pulses at a repetition rate of 100 MHz while the lasers are subjected to external optical injection. Under these conditions, broad and Hat optical frequency combs (OFC) are generated, showing 500 tones within 10 dB and a line spacing fix ed by the repetition rate of the swi tching pulses. The dual-comb gene rated by th e interference of two such combs with slightly different line spacing, is used for measuring the absorption line of CO2 at 1572.02 nm. The transmission spectrum is analyzed and fitted to a Voigt profile. A deviation of 3% in the residuals of the fitting is obtained, denoting the high spectral performance of the spectroscopy system.

Multi-Band Triangular Frequency Modulation Signal Generation Based on Gain-Switched Laser

Multi-Band Triangular Frequency Modulation Signal Generation Based on Gain-Switched Laser

Single-frequency linearly-polarized gain-switched DFB pulsed fiber laser

We report a single-frequency linearly-polarized gain-switched distributed feedback (DFB) pulsed fiber laser. A 976 nm laser diode (LD) which can generate both pulsed and continuous wave (CW) laser is used as the pump source to initiate gain-switched progress. Pulsed fiber laser with single-frequency and linearly-polarization is obtained by applying pressure on a 50 mm long phase shifted fiber Bragg grating (PS-FBG). Through parameter optimization, the laser generates single-frequency linearly-polarized laser pulses with a center wavelength of 1030.78 nm, a narrowest linewidth of 36.1 MHz, a maximum peak power of 3.78 W, and a polarization extinction ratio of better than 19 dB. And the repetition rate of output pulses can be flexibly changed from 1 kHz to 400 kHz with a pulse width of 20–200 ns.

Quantum random number generator based on polarization switching in gain-switched VCSELs

We experimentally study a quantum random number generator based on the random excitation of the linearly polarized modes of a gain-switched vertical-cavity surface-emitting laser (VCSEL). Our device is characterized by having polarization switching under continuous wave operation. By measuring the linear polarization mode that is excited in each pulse we collect a sufficient number of bits to evaluate if a standard statistical test suite is passed. We consider linear and Von Neumann post-processing methods in order to reduce the bias with different levels of bits rejection. The post-processed bit strings pass all tests in the standard test suite for random number generators provided by the National Institute of Standards and Technology (NIST). We finally compare the results obtained with different post-processing functions, including several [n, k, d] linear BCH codes. We show that large values of n and k are the best choice to obtain simultaneously improved throughput and randomness.

Theoretical analysis and experimental demonstration of gain switching for a PPM based UWOC system with picosecond pulses.

Shortening pulse width can improve the power efficiency and data rate of a pulse position modulation (PPM) based underwater wireless optical communication (UWOC) system at a fixed average optical power, which is more suitable for the energy-limited underwater environment. As a common method to generate short pulses, gain switching has the advantages of a tunable switching frequency and simple structure, facilitating the generation of high-order PPM signals. However, the output characteristics of electrical gain switching seriously affect the demodulation of PPM signals and limit the data rate. To study the performance of gain switching on a PPM communication system, simulation models of the semiconductor laser diode and the driving circuit are built to describe the generation of electrical and optical pulses. The pulse width, pulse peak value, and peak position of optical pulses are analyzed under different symbol durations and PPM orders. Furthermore, a 64-PPM/150-Mbps UWOC system with a 200-ps optical pulse width is demonstrated by using a gain-switched blue GaN-based laser diode in a water tank. The peak average power ratio (PAPR) is 19.5 dB. Via the statistical analysis of experiment results and the output characteristics of electrical gain switching, the main factor limiting the data rate attributes to the time delay fluctuation of gain switching. To the best of our knowledge, this is the first time that gain switching has been experimentally demonstrated and analyzed in a high-order PPM based UWOC system.

Numerical simulation of the magnetically gain-switched chemical oxygen-iodine laser

During the operation of the magnetically gain-switched chemical oxygen-iodine laser (MGS-COIL), the transition intensity of hyperfine transition line 2-2 can exceed that of line 3–4, which is the dominant line at zero magnetic field. For this reason, a simulation model including both 3–4 and 2-2 transition lines is necessary to describe the mode buildup process in MGS-COIL. In this paper, we assume that 3–4 and 2-2 transition lines simultaneously oscillate in laser cavity. The propagation of optical field is calculated based on FFT. The required frequency, rise time and residual field of the magnetic gain-switch for a high-performance MGS-COIL are analyzed based on simulation results.

Towards generation of indistinguishable coherent states

We have addressed the characteristic distinguishability of coherent states in the temporal domain from a directly modulated quantum well-based gain-switched laser diode. We identify adjustable parameters to generate indistinguishable coherent states from an electrically pumped semiconductor laser using small-signal and large-signal models. The experiment confirms the generation of indistinguishable signal and decoy coherent states as predicted by the numerical simulation. In addition, the potential for indistinguishability has been explored in different types of coherent states.

Analytical theory of gain-switched laser pulses: From Lotka-Volterra slow-fast dynamics to passive Q switching.

Gain switching is a simple technique for generating short pulses through direct modulation of optical gain in lasers. Its mathematical description requires the connection between a slowly varying, low intensity solution and a short, high intensity solution. Previous studies constructed a complete pulse by patching these two partial solutions at an arbitrary point in the phase plane. Here, we develop an asymptotic theory in which slow and fast solutions are matched together through a third intermediate solution. The mathematical analysis of the laser problem benefits from a preliminary study of the Lotka-Volterra equationswhen the two competing populations exhibit different timescales. Since this particular limit has never been explored, we first analyze the Lotka-Volterra equationsbefore applying the theory to the more complex laser equations. We show a significant effect of the transition layer on the pulse intensity. Last, we discuss the case of sustained laser pulses generated through the Q-switching technique and show how their description may benefit from our theory.

Pulse-Profile-Maintaining Characteristics of Gain-Switched Nanosecond Raman Fiber Laser and Amplifier

This paper investigates pulse-profile-maintaining characteristics of a gain-switched nanosecond Raman fiber laser (RFL) and its Raman fiber amplifier (RFA). It is found that the pulse profiles can be precisely maintained after the gain-switched Raman conversion, only if the pump pulse repetition frequency (PRF) is an integer multiple of, a unit fraction of, or multiple unit fractions of the fundamental cavity-defined PRF. The produced Raman pulses are then amplified in an RFA, and the pulse profiles can also be precisely maintained. However, it is further noticed that the detuning-induced defects can be amplified if they are with the pulse itself. All these observed phenomena should originate from the transient property of Raman gain, not involving any notable delaying or saturation effects that are typically seen with rare-earth-doped gain fibers. Our results demonstrate that, in the nanosecond scale, the Raman-converted pulse can well maintain its profile and shows strong resistance to detune between PRFs of the pump and Raman pulses. Benefiting from that, we can reasonably neglect the pump depletion effect due to walk-off effect typically seen in various short pulse regimes, which can largely simplify the design of nanosecond pulsed pumping Raman sources.