High power 2-μm room-temperature continuous-wave operation of GaSb-based strained quantum-well lasers

A high power GaSb-based laser diode with lasing wavelength at 2 μm was fabricated and optimized. With the optimized epitaxial laser structure, the internal loss and the threshold current density decreased and the internal quantum efficiency increased. For uncoated broad-area lasers, the threshold current density was as low as 144 A/cm2 (72 A/cm2 per quantum well), and the slope efficiency was 0.2 W/A. The internal loss was 11 cm-1 and the internal quantum efficiency was 27.1%. The maximum output power of 357 mW under continuous-wave operation at room temperature was achieved. The electrical and optical properties of the laser diode were improved.     

Generation of optical waveforms in 1.3-μm SOA-based fiber lasers

Passive optical waveform generation is obtained in fiber lasers using a 1.3-μm semiconductor optical amplifier (SOA) as the gain medium. Various waveforms, including square wave, staircase wave, triangular wave, pulse, and dark pulse are generated in SOA-based fiber lasers by adjusting intracavity polarization controllers. The passive waveform generation might be attributed to the SOA gain dynamics and the enhanced nonlinear interaction at the 1.3-μm zero dispersion wavelength of traditional single-mode fiber (SMF), as well as the interference effect between the two sub-cavities of fiber laser. With figure-8 cavity configuration, 1250th-order harmonic pulses have been successfully demonstrated. We have also obtained a free-running SOA-based fiber laser with 3-dB spectral width of 16 nm, and the center wavelength can be tuned over 45 nm range.     

Broadband terahertz light source pumped by a 1 μm picosecond laser

We obtained a frequency tunable, low-coherence, picosecond, terahertz (THz) output with a high repetition rate from a picosecond Nd:YVO₄ bounce laser in combination with tandem periodically poled stoichiometric lithium tantalate and 4′-dimethylamino-N-methyl-4-stilbazolium tosylate crystals. The frequency of the THz output was tunable in the range 2.1–7.1 THz with a linewidth of ~3.5 THz at 2.2 THz. The THz output had a maximum peak power of ~180 mW and an average power of ~0.65 μW at 3.9 THz. This system has the potential to realize ultra-high speed, THz coherence tomography.     

Systematic study on the confinement structure design of 1.5-μm InGaAlAs/InP multiple-quantum-well lasers

An in-dept analysis on the separate confinement heterostructure (SCH) design parameters of 1.5-μm InGaAlAs/InP multiple quantum-well (MQW) lasers is reported. Theoretical calculations show a drastic enhancement on threshold current and slope efficiency from step-index SCH (STEP-SCH) to graded-index SCH (GRIN-SCH) design, but the effect ceases beyond a critical number of grading steps. This finding implies ease of the growth process and reduction in cost. The overall GRIN-SCH’s thickness is found to exert great influence over the achievable laser’s threshold current and slope efficiency. An average of 27 mA threshold current reduction and more than 32% of slope efficiency increment were achieved by optimizing the GRIN-SCH thickness. Increasing the grading energy range of the GRIN-SCH decreases the slope efficiency, but is found to effectively reduce carrier leakage at elevated temperature leading to a less temperature-sensitive threshold current MQW ridge lasers were fabricated and characterized out of two laser materials, one with a reference STEP-SCH and another with a proposed optimized GRIN-SCH profile. The laser with optimised SCH design has shown a 36% reduction in room temperature threshold current as compared to that with the STEP-SCH design, which is in good agreement to the theoretical simulation. In addition, a record high characteristic temperature (T ₀) of 105 K was obtained on the GRIN-SCH laser structure, which is more than three fold increment as compared to the STEP-SCH design.     

High-power source of coherent picosecond light pulses tunable from 0.41 to 12.9 μm

We report a high-power source of coherent picosecond light pulses based on optical parametric generation and amplification in LiB₃O₅ and AgGaS₂ crystals. The spectral range of this continuously tunable source covers the visible, near-infrared and medium-infrared spectrum from 0.41 to 12.9 m. An optical parametric generator and amplifier, consisting of two type-I phase-matched LiB₃O₅ crystals and a diffraction grating, is pumped by the third harmonic of a picosecond Nd:YAG laser and provides spectrally narrow, high-power pulses from 0.41 to 2.4 m. Energy conversion efficiencies up to 16 percent are achieved. The pulse duration is about 14 ps, the bandwidth between 10 and 30 cm^–1. The tuning range is extended to 12.9 m by mixing the infrared output between 1.16 and 2.13 m with the fundamental of the Nd:YAG laser in type-I-phase-matched AgGaS₂ crystals. Up to 25 percent of the pulse energy at 1.064 m is converted into parametric infrared pulses. Bandwidths between 3 and 8 cm^–1 and a pulse duration of approximately 19 ps are measured for these pulses. We also observe a retracing behaviour in the tuning curve of AgGaS₂ not reported before.     

Research on 2-μm solid-state lasers

Diode-pumped solid-state 2 μm lasers have rapid development for their efficient, compact and stable performance. In this paper, we will introduce and discuss the work on 2 μm solid-state-lasers. The advantages and disadvantages of four ways to realize 2 μm laser output are generalized and discussed.     

PbS-doped phosphate glasses saturable absorbers for 1.3-μm neodymium lasers

Passive mode locking and saturable absorber Q-switching of neodymium lasers at 1.3 μm with PbS-doped phosphate glasses are demonstrated. Q-switched pulses of 120 ns (0.1 μJ) in duration (energy) and the average output power of 3 mW from a quasi-cw diode-pumped Nd³⁺:KGW laser and ultrashort pulses of a maximum of 250 μJ in energy and 150 ps in duration from a Nd³⁺:YAP laser were obtained. The bleaching decay rate of the samples was found to increase with the Quantum Dot’s size decreasing due to the enhancement of quantum confinement effects for smaller dots and stronger overlapping of the electron and trap state wave functions. Received: 23 January 2002 / Revised version: 2 April 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +375-17/232-6286, E-mail: savitski@eudoramail.com     

High output power operation of 1.3-μm strained MQW lasers with low threshold currents at high temperature

We have investigated the temperature dependencies of the slope efficiency and the threshold current for strained multiquantum well (MQW) lasers as a parameter of the well number. Smaller well numbers mean larger temperature dependencies of the slope efficiency and the threshold current, while larger well numbers mean larger internal loss and broadening of the photoluminescence linewidth of the MQW structure. Furthermore, the change in the slope efficiency with temperature change is related to the change in internal loss. In this work, the 1.3-m strained MQW laser with a compressive strain of 1.0% and 7 wells shows the highest output power of 6.8 mW for an injection current of 50 mA and the lowest threshold current of 5.5 mA at 85°C, and the lowest variation in output power of 2.0 dB from 25–85°C at injection current of 50 mA.     

Ho:LaF3 single crystal as potential material for 2 μm and 2.9 μm lasers

Optical properties of a Ho-doped LaF₃ single crystal have been detailed investigated as a promising material for 2 μm and 2.9 μm lasers for the first time. Judd–Ofelt theory was applied to analyze the absorption spectrum to determine the J–O intensity parameters Ω_t(t=2,4,6), based on which the emission probabilities, branching ratio and radiative lifetime for the as-grown crystal were all calculated. The stimulated emission cross-sections of the ⁵I₇ → ⁵I₈ and ⁵I₆ → ⁵I₇ transitions were obtained by using the Fuchtbauer–Ladenburg method. The gain cross-section for 2 μm emission becomes positive once the population inversion level reaches 30%. The Ho:LaF₃ crystal shows long fluorescence lifetime of ⁵I₇ manifold (25.81 ms) as well as ⁵I₆ manifold (10.37 ms) compared with other Ho³⁺-doped crystals. It can be proposed that the Ho:LaF₃ crystal may be a promising material for 2 μm and 2.9 μm laser applications.     

1.55-μm Si-based MOEMS optical tunable filter

A prototype 1.55-μm Si-based micro-opto-electro-mechanical-systems (MOEMS) tunable filter is fabricated, employing surface micromachining technology. Full-width-at-half-maximum (FWHM) of the transmission spectrum is 23 nm. The tuning range is 30 nm under 50-V applied voltage. The device can be readily integrated with resonant cavity enhanced (RCE) detector and vertical cavity surface emitting laser (VCSEL) to fabricate tunable active devices.     

2-μm singly resonant optical parameter oscillator

We experimentally realize a singly resonant optical parameter oscillator （OPO） which operates at the degenerate point of KTiOPOa （KTP） crystal. A thin film plate polarizer （TFP） is used as the output coupler to obtain linearly polarized 2.12 μm laser. A maximum output power of 2.2 W at 2.12 μm is obtained from a near-degenerate, type-II KTP OPO by utilizing a Q-switched diode-pumped Nd：YAG laser operating at 5 kHz.     

High-power 3.8 μm tunable optical parametric oscillator based on PPMgO:CLN

The experimental results of a high-power 3.8 μm tunable laser are presented on a quasi-phase-matched single-resonated optical parametric oscillator in PPMgO:CLN pumped by a 1064 nm laser of an elliptical beam. Theoretical analyses of the PPMgO:CLN wavelength tuning are presented. The pump source was an acousto-optical Q-switched cw-diode-side-pumped Nd:YAG laser. The beam polarization matched the e-ee interaction in PPMgO:CLN. When the crystal was operated at 90 °C and the pump power was 150 W with a repetition rate of 10 kHz, average output power of 22.6 W at 3.86 μm and 63 W at 1.47 μm was obtained. The slope efficiency of the 3.86 μm laser with respect to the pump laser was 17.8%. The M² factors of the 3.86 μm laser were 1.74 and 4.86 in the parallel and perpendicular directions, respectively. The mid-IR wavelength tunability of 3.7-3.9 μm can be achieved by adjusting the temperature of a 29.2 μm period PPMgO:CLN crystal from 200 °C to 30 °C, which basically is accorded with the theoretic calculation.     

Demonstration of fiber pulsed light source at 1.6μm with adjustable pulse duration

A novel practical 1.66-μm pulse light source with adjustable pulse duration is proposed. A 2.5-km Raman fiber is placed into a ring type Q-switched erbium-doped fiber laser (Q-EDFL), serving as both delay line fiber and Raman gain medium so that in addition to the wavelength shifted to 1.6μm, the pulse duration and the buildup time can be relatively extended. By properly controlling the fall edge of the acousto-optic switch (AOS), the pulse duration of 30-345 ns for～770-Hz repetition frequency with power of 1-1.6 W is achieved.     

1.55 μm distributed feedback (DFB) lasers subject to strong 1.4 μm optical injection

An experimental analysis of the influence of optical injection at 1.4 μm wavelength into two different commercial 1.55 μm DFB lasers is reported. The results demonstrate the strong dependence of the DFB behaviour on the injection parameters. Complete mode suppression or signal amplification can be obtained by varying the excitation wavelength and/or intensity, suggesting that these devices could be operated as logic ports or signal amplifiers, according to the injected signal.     

High efficiency beam combination of 4.6-μm quantum cascade lasers

The quantum cascade laser （QCL）, a potential laser source for mid-infrared applications, has all of the advantages of a semiconductor laser, such as small volume and light weight, and is driven by electric power. However, the optical power of a single QCL is limited by serious self-heating effects. Therefore, beam combination technology is essential to achieve higher laser powers. In this letter, we demonstrate a simple beam combination scheme using two QCLs to extend the output peak power of the lasers to 2.3 W. A high beam combination efficiency of 89% and beam quality factor of less than 5 are also achieved.     

Holmium Oxide Film as a Saturable Absorber for 2μm Q-Switched Fiber Laser

This work reports on the use of the holmium oxide(Ho_2 O_3) polymer film as a saturable absorber(SA) for generating stable Q-switching pulses operating in a 2-μm region in a thulium-doped fiber laser cavity. The SA is prepared by diluting a commercial Ho_3 O_2 powder and then mixing it with polyvinyl alcohol(PVA) solution to form a Ho_2 O_3-PVA film. A tiny part of the film about 1 mm×1 mm in size is sandwiched between two fiber ferrules with the help of index matching gel. When incorporated in a laser cavity driven by a 1552-nm pump,stable Q-switching pulses are observed at 1955 nm within the pump power range of 363-491 mW. As the pump power increases within this range, the repetition rate rises from 26 kHz to 39 kHz, as the pulse width drops from4.22μs to 2.57μs. The laser operates with a signal-to-noise ratio of 47 dB, and the maximum output power and the pulse energy obtained are 2.67 mW and 69 nJ, respectively. Our results successfully demonstrate that the Ho_2 O_3 film can be used as a passive SA to generate a 2-μm pulse laser.