Sub-nanosecond Nd:Lu0.61Gd0.39VO4 microchip laser

We report the fabrication and operation of a single-to-three-mode, sub-nanosecond passively Q-switched Nd:Lu_0.61Gd_0.39VO₄/Cr⁴⁺:YAG microchip laser, which exhibits changes in mode structure with increasing incident pump power. The laser exhibits longitudinal mode oscillations with a partial transverse mode overlap. The shortest pulse duration, highest pulse energy and peak power observed are 646 ps, 8.7 μJ and 13.5 kW, respectively.     

Microchip lasers and their applications in optical microsystems

The microchip lasers are the most compact diode pumped solid state lasers. They are fabricated using collective mass production processes, at low cost. Several hundreds of microchip lasers can be fabricated on one single thin wafer of a laser material. The microchip laser is pumped with a standard GaAlAs or GaInAs diode laser, directly or through a multimode fibre. It is a kind of optical transformer which transforms a poor quality laser diode beam to a diffraction limited TEM00 and single frequency laser beam. Moreover, by Q-switching, very short pulses (∼0.4–2 ns) with very high peak power (0.5–50 kW) can be obtained. Most of the well known laser materials can be used for emissions near 1, 1.3, 1.5 and 2. Visible and UV emissions are obtained using harmonic generations in non-linear crystals. Microchip lasers have many different industrial applications in large markets such as: automotive, laser marking, environmental and medical applications, public works, telecommunications. They should open the domain of solid state lasers to high volume and low cost markets.     

High-power,high-repetition-rate picosecond optical parametric oscillators for the near-to mid-infrared

Abstract

We report high-repetition-rate, singly-resonant, picosecond optical parametric oscillators based on the nonlinear crystals LiB₃O₅ and KTiOAsO₄ which are synchronously pumped by a self-mode-locked Ti:sapphire laser operating at 81 MHz. These devices allow tunable pulse generation from 1·116-3·160 μm to be achieved. The LiB₃O₅ system produces average nearinfrared output powers of 325 mW and is continuous tuning over the wavelength range 1·16-2·26 μm. For 1·8 ps input pump pulses, transform-limited signal pulses with durations of 1-1·2 ps and idler pulses with durations of 2-2·2 ps have been generated over 1·2-2·2 μm, without requirement for dispersion compensation. The KTiOAsO₄ system produces average near-infrared output powers of 403 mW, with the signal tuning over 1·116-1·281 μm and idler tuning over 2·260-3·160 μm. Without dispersion compensation, signal (idler) pulses with durations between 1·01-1·03 (1·61-2·91) ps have been obtained for 1·2 ps input pump pulses.     

Generation of high-power ultrashort optical pulses by semiconductor lasers

Fiber-coupled semiconductor lasers have been studied when pumped by high-power short electrical pulses of 5 ns width and leading front duration below 1 ns. In this pumping regime, it is possible to ensure significant sharpening of output pulses, the duration of which decreases below 80 ps for a single-mode laser and below 120 ps for a broad aperture multimode laser at an output peak optical power as high as 1.5 and 27 W, respectively.     

Fabrication of Al/MgO/C and C/MgO/InSe/C tunneling barriers for tunable negative resistance and negative capacitance applications

In this work, the design and characterization of magnesium oxide based tunneling diodes which are produced on Al and InSe films as rectifying substrates are investigated. It was found that when Al thin films are used, the device exhibit tunneling diode behavior of sharp valley at 0.15 V and peak to valley current ratio (PVCR) of 11.4. In addition, the capacitance spectra of the Al/MgO/C device show a resonance peak of negative capacitance (NC) values at 44.7 MHz. The capacitance and resistance–voltage characteristics handled at an ac signal frequency of 100 MHz reflected a build in voltage (V_bi) of 1.29 V and a negative resistance (NR) effect above 2.05 V. This device quality factor (Q)–voltage response is ~10⁴. When the Al substrate is replaced by InSe thin film, the tunneling diode valley appeared at 1.1 V. In addition, the PVCR, NR range, NC resonance peak, Q and V_bi are found to be 135, 0.94–2.24 and 39.0 MHz, ~10⁵ and 1.34 V, respectively. Due to the wide differential negative resistance and capacitance voltage ranges and due to the response of the C/MgO/InSe/C device at 1.0 GHz, these devices appear to be suitable for applications as frequency mixers, amplifiers, and monostable–bistable circuit elements (MOBILE).     

Single-mode, tunable output from a midwave-infrared-seeded optical parametric oscillator system

We have built a coupled master oscillator and a slave oscillator optical parametric oscillator (OPO) system that provides single-frequency, pulsed radiation in the midwave infrared (MWIR). The direct-diode-pumped master OPO provided narrow-band (<6-MHz, instrument-limited), tunable MWIR cw radiation that was used to seed a higher-peak-power pulsed slave OPO. When seeded directly at the MWIR idler, the pulsed output of the slave OPO was constrained to oscillate on a single longitudinal mode even though the slave OPO is pumped by a multilongitudinal-mode laser source. The linewidth of the pulsed output has been measured to be <220 MHz (instrument limited), which is well suited for coherent differential absorption lidar applications.     

Narrow-linewidth passively Q-switched fibre laser based on microsphere resonator

We experimentally demonstrate a stable narrow-linewidth passively Q-switched fibre laser based on a microsphere resonator (MSR) and graphene saturable absorber (SA). The MSR made by the arc discharge method has the characteristics of easy fabrication, broad free spectral range (FSR) and flexibility. And it acts as a narrow band-pass filter to ensure narrow-linewidth operation. A stable passively Q-switched pulse with 0.016?nm narrow spectral linewidth is successfully achieved. The output pulse has the pulse width of 5.2 µs, repetition frequency of 28 kHz and high signal to noise ratio (SNR) of ～60?dB. The results demonstrate that our works may provide an effective way to achieve narrow-linewidth pulsed fibre lasers.     

Evaluation of quantum-cascade lasers as local oscillators for infrared heterodyne spectroscopy

We report experiments evaluating the feasibility of quantum-cascade lasers (QCLs) at mid-infrared wavelengths for use as local oscillators (LOs) in a heterodyne receiver. Performance tests with continuous-wave (cw) lasers around 9.6 and 9.2 microm were carried out investigating optical output power, laser linewidth, and tunability. A direct comparison with a CO2 gas laser LO is presented as well. The achieved system sensitivity in a heterodyne spectrometer of only a factor of 2 above the quantum limit together with the measured linewidth of less than 1.5 MHz shows that QCLs are suitable laser sources for heterodyne spectroscopy with sufficient output power to replace gas lasers as LOs even in high-sensitivity astronomical heterodyne receivers. In addition, our experiments show that the tunability of the lasers can be greatly enhanced by use of an external cavity.     

Doppler-free spectroscopy using a continuous-wave optical frequency synthesizer

A continuous-wave (cw) optical frequency synthesizer is demonstrated by using a monolithic-type cw optical parametric oscillator (cw-OPO) and an optical frequency comb. The cw-OPO is phase locked to an optical frequency comb that is phase locked to an atomic clock. The output frequency of the cw-OPO is frequency shifted with an electro-optic modulator, which makes it possible to tune the frequency continuously over 10 GHz. Furthermore, Doppler-free spectroscopy is performed using the optical frequency synthesizer for a cesium D1 line at 895 nm. The observed linewidth of 5 MHz is the natural linewidth of cesium. The center frequency of the line is consistent with a previous report.     

AM and high-harmonic FM laser mode locking

We demonstrate a new technique of active mode locking that combines amplitude-modulated (AM) mode locking at the cavity fundamental repetition rate with frequency-modulated (FM) mode locking at a high harmonic. This method combines the advantages of pulse shortening by high-harmonic mode locking while preserving the higher peak powers available at the fundamental repetition rate. We demonstrate this technique using a Nd:YAG laser that is simultaneously AM mode locked at 80 MHz and FM mode locked at the 22nd harmonic (1.76 GHz). Pulses as short as 16 ps with a peak power of 6.25 kW were measured.     

Compact solid state radio frequency amplifiers in kW regime for particle accelerator subsystems

Radio frequency and Microwave (RFM) infrastructure test facility is under development at RRCAT for evaluating and powering, subsystems of particle accelerator. As a part of this facility, design of 20–30 kW UHF solid state power amplifiers is in progress. For this work, design procedure has been formulated for the development of solid state amplifier modules, radial combiner, divider and directional coupler; with specifications suited to RFM power system for particle accelerator. Methodology has been demonstrated by developing two different compact amplifiers with power output of 2 kW each, operating at 352 MHz and 505.8 MHz, respectively. This paper describes underlying design principles and indigenous development of these amplifiers, consisting of 270–300 W amplifier modules, 8-way 2 kW radial combiner/divider and directional couplers. Design methodology for power combiner has been extended by physically realizing higher power (4 kW) 16-way power combiner and 2-way combiner (8 kW) for higher power (8 kW) amplifier configuration planned. Simple design, indigenous technology, high efficiency and ease of fabrication, are the main features of this design.     

Spatial and temporal dependence of single-pass parametric gain

Abstract

The use of focused Gaussian beams and short optical pulses to pump single-pass optical parametric amplifiers is examined. A simple model is introduced to mimic the experimental conditions used, and the expected intensity profile of the output signal for degenerate parametric amplification in LBO and MgO:LiNbO₃ is calculated. Experimental and theoretical results are compared for LBO. Second harmonic autocorrelation measurements were performed in order to determine the pulse duration of the output signal from the LBO parametric amplifier. These measurements showed that the amplified pulse was slightly shorter, and the deamplified pulse slightly longer, than the input signal—a result of the variation in the parametric gain over the 3 ps signal pulse. The implications of this spatial and temporal variation of the gain in squeezing are discussed.     

Ultrafast Yb:Y₂SiO₅ laser investigation based on a carbon nanotube absorber

Transmission-type single-walled carbon nanotube saturable absorbers were successfully fabricated and used in a CW passively mode-locked Yb:Y?SiO? laser for the first time to our knowledge. We obtained pulses as short as 1.1 ps around a center wavelength of 1058 nm. The average output power of 1.1 W was achieved at the repetition of 96.7 MHz; the corresponding peak power and energy of a single pulse was 10.3 kW and 11.4 nJ, respectively.     

Mid-infrared w quantum-well lasers for noncryogenic continuous-wave operation

We review the recent progress of electrically injected and optically pumped mid-IR lasers based on antimonide quantum wells with the type II W configuration. W quantum-well diodes have achieved cw operation up to 195 K at lambda = 3.25 mum. Optically pumped devices that employ the diamond pressure bond heat sink have reached 290 K at 3 mum and 210 K at 6 mum. Pulsed power conversion efficiencies of up to 7% at 220 K have been attained by use of an optical pumping injection cavity approach, in which an etalon cavity for the pump beam significantly enhances its absorptance. The angled-grating distributed-feedback configuration has been used to obtain near-diffraction-limited output for an optical pumping stripe width of 50 mum.     

Passive mode-locking of diode-pumped Nd:GdVO4 laser with carbon nanotube intracavity absorber

By using single-walled carbon nanotubes as the saturable absorber fabricated by a vertical evaporation method, passive mode-locking of a Nd:GdVO₄ laser was realized in a W-type folded cavity. The laser generated 8?ps pulses at a repetition rate of 81?MHz. At 10.4?W of the incident pump power, average output power of 1.22?W was achieved and the corresponding peak power and energy of a single pulse were 1.88?kW and 15?nJ, respectively.     

Extremely low phase noise UHF oscillators utilizing high-overtone, bulk-acoustic resonators

High-overtone, bulk acoustic resonators (HBAR) have been designed that exhibit 9-dB insertion loss and loaded Q values of 80000 at 640 MHz with out-of-phase resonances occurring every 2.5 MHz. These resonators have been used as ovenized frequency-control elements in very low phase noise oscillators. The oscillator sustaining stage circuitry incorporates low-1/f noise modular RF amplifiers, Schottky-diode ALC, and a miniature 2-pole helical filter for suppression of HBAR adjacent resonant responses. Measurement of oscillator output signal flicker-of-frequency noise confirms that state-of-the-art levels of short-term frequency stability have been obtained. Sustaining stage circuit contribution to resulting oscillator flicker-of-frequency noise is 7-10 dB below that due to the resonators themselves. At 16-dBm resonator drive, an oscillator output signal white phase noise floor level of -175 dBc/Hz is achieved.     

Quantum-dot semiconductor optical amplifier performance management under optical injection

The effect of an external beam on different characteristics of quantum-dot semiconductor optical amplifiers (QD-SOAs) has been investigated experimentally. It has been shown that injection of a relatively weak light beam at the gain transparency wavelength accelerates the full gain recovery time from 80 ps to 65 ps while it increases the saturation output power (SOP) by more than 2 dB if the beam resides in the gain region. The effect of the external beam on the fiber-to-fiber gain has been demonstrated at both the gain transparency wavelength and close to gain peak wavelength. These results are expected to be further enhanced by increasing the injected beam power, which has a peak value of ～7 mW in the experiments.     

Pulsed optical parametric amplification based on photonic crystal fibres

In this paper, we simulate pulsed one-pump fibre optical parametric amplifiers (1-P FOPAs) based on photonic crystal fibres in telecommunication region. At first, a PCF is designed such that its zero-dispersion wavelength is located at 1535 nm in order to fulfil the phase-matching condition for the parametric amplification in the telecommunication region. Then, the dispersion coefficients of the PCF including β₂, β₃, β₄ and β₅ as well as the nonlinear parameters are calculated at the pump, signal and idler wavelengths. Finally, the coupled nonlinear Schrödinger equations are numerically solved and the impacts of pump power and the pulse shape on the evolution of each pulse along the 1-P FOPA are investigated. The results show that the peak powers of the pump, signal and idler are periodically varied and each pulse is distorted and broadened as it propagates down the fibre optical parametric amplifiers. Also, the pulse distortion and broadening increase with the increase in both propagation distance and the input peak power.     

Crystalline-silicon-based infra-red LEDs and routes to laser diodes

We review progress in silicon LEDs using dislocation engineering to achieve high temperature operation, a process that is fully CMOS (Complementary Metal Oxide Semiconductor) compatible. We concentrate on devices operating in the near infra-red where high value applications are. The need for silicon emitters, lasers and optical amplifiers is discussed followed by an outline of previous approaches and possible future routes explored. Results on gain in silicon are reported and routes to electrically pumped injection lasers and optical amplifiers considered. Extension of 1.1 and 1.5 μm devices to other wavelengths is discussed.     

Electrically pumped waveguide lasing from ZnO nanowires

Ultraviolet semiconductor lasers are widely used for applications in photonics, information storage, biology and medical therapeutics. Although the performance of gallium nitride ultraviolet lasers has improved significantly over the past decade, demand for lower costs, higher powers and shorter wavelengths has motivated interest in zinc oxide (ZnO), which has a wide direct bandgap and a large exciton binding energy. ZnO-based random lasing has been demonstrated with both optical and electrical pumping, but random lasers suffer from reduced output powers, unstable emission spectra and beam divergence. Here, we demonstrate electrically pumped Fabry-Perot type waveguide lasing from laser diodes that consist of Sb-doped p-type ZnO nanowires and n-type ZnO thin films. The diodes exhibit highly stable lasing at room temperature, and can be modelled with finite-difference time-domain methods.