We report a passively Q-switched two-component visible laser light source based on frequency conversion. The device
consists of a monolithic single transverse mode ridge-waveguide infrared laser diode and a waveguide-type periodically
poled magnesium oxide doped lithium niobate crystal for second harmonic light generation. An integrated 45-degree
folding mirror and a coupling lens are formed by etching on opposite sides of the monolithic gallium arsenide -based
laser diode for coupling the infrared emission into the waveguide-type nonlinear crystal for efficient single pass
frequency conversion. Passively Q-switched operation is realized by an integrated electro-absorber section coupled with
the in-plane multi-quantum well gain structure. Stable high-repetition rate self-pulsating operation was achieved by
reverse-biasing the electro-absorber section and reduced speckle visibility of the second harmonic light was observed
when compared to continuous-wave operation of the same laser.
We report a fast-switching two-component frequency-converted laser with reduced speckle visibility. A bottom-emitting
passively Q-switched laser with integrated electroabsorber, folding mirror and coupling lens was successfully applied
with a waveguide-type periodically poled magnesium oxide doped lithium niobate crystal to generate second harmonic
light at 532 nm. Reduced speckle visibility was demonstrated when operating the laser in self-pulsating mode when
compared to continuous-wave operation even when using a nonlinear crystal with narrow acceptance bandwidth of 0.24
nm @ 1064 nm. The spectral width of the infrared light was 0.33 nm in pulsed mode and 0.08 nm in continuous-wave
mode resulting in visible light spectral width of 0.12 nm and 0.03 nm in self-pulsating and continuous-wave mode.
At the present time, there is a considerable demand for long wavelength (1.3μm-1.5μm) laser diodes for low cost data-communication applications capable of operating at high speed and at high ambient temperatures without the need for thermoelectric coolers. First proposed in 1995 by M. Kondow, the GaInNAs/GaAs material system has attracted a great deal of interest as it promises good temperature performance. The broad gain observed in GaInNAs/GaAs QW samples suggests that wavelength tuning should be possible by the application of gratings to select an optical mode. In addition, splitting the contact has been shown to improve modulation speed in other materials. These two methods should be able to be used jointly and processed together. The use of split-contact lasers has the advantage of that no change is made in the processing steps, since there is only need for a new metal mask to define a new top p-contact. Despite the bandwidth enhancement of two-contact lasers compared to the single contact case is well known, to the authors' knowledge, so far it has not being applied to GaInNAs/GaAs lasers. The use of Bragg-gratings on the ridge waveguide of the laser will generate a periodic modulation inducing an interaction between the forward and backward travelling modes. The effect of this interaction is the one of a band pass filter on the gain shape of the laser, allowing filtering out the actual lasing wavelength, and tuning the lasing wavelength in the range of wavelengths with substantial optical gain. Therefore, this method can be used optimally in lasers with broad gain, as is the case of GaInNAs/GaAs. In this paper, we reveal experimental investigations in how to apply these two post-processing methods to 600m-long 1.25μm-Ga<sub>0</sub>.66In<sub>0</sub>.34N<sub>0</sub>.01As0.<sub>0</sub>.99/GaAs 6nm single quantum-well ridge waveguide lasers.
Beryllium incorporation in InGaAsN quantum well improves the optical properties of this dilute nitride material significantly. After annealing, the intensity of the photoluminescence of this new dilute nitride material (InGaAsNBe) is about 20 times higher and its wavelength is even 25 nm longer. After a certain time of this heat treatment, the photoluminescence quenched slowly for InGaAsN structures because of the strain relaxation due to the thermal activation. The photoluminescence of InGaAsNBe increased rapidly and show no saturation even after a very long time of annealing. Beryllium incorporation in InGaAs which was grew at the same temperature as dilute nitrides also improves the optic properties. But the improvement for InGaAsNBe is 10 times more than for InGaAsBe. Laser processing based on the new InGaAsNBe structures resulted in one half of the threshold current density compare to conventional InGaAsN.
Beryllium was incorporated in InGaAsN single quantum well (SQW). Comparing with the conventional InGaAsN SQW structures, photoluminescence (PL) investigations show a significant improvement. After 3000 sec of annealing at 700 °C, the PL peak area is about 20 times higher while the wavelength keeps 25 nm longer. After 800 sec of this annealing, the PL quenched slowly for the conventional structures because of the strain relaxation, while the PL of the new structures increased rapidly and show no saturation after 3000 sec of annealing. Laser processing based on the new InGaAsN structures resulted in one half of the threshold current density compare to conventional InGaAsN.
The potential of 1.3um GaInNAs SQW laser diodes for high speed operation is experimentally investigated in this paper, computing the differential gain, dg/dn, at a temperature range suitable for most network applications (293K-348K) and the small signal modulation bandwidth. The investigation begins with a basic characterization calculating the T0, with a value of 56K in a range of temperatures of 293K-318K. The lasing wavelength at 293K is found to be 1250nm with a linear temperature dependence of 0.377nm/K. Secondly, the paper presents a detailed study of the modulation bandwidth of the device, obtaining a value of 6.06Ghz for the maximum modulation bandwidth at 293K. In a range of temperatures of 293K-318K, the modulation bandwidth is found to decrease only slightly with the temperature with a slope of 0.0088Ghz/K. Finally, the paper explains the temperature behaviour obtained for the modulation bandwidth studying the temperature dependence of the differential gain, dg/dn. For this evaluation, the value of the differential gain with the current (how the peak gain changes with the sub-threshold bias current applied to the sample), dg/dI, is obtained using the Hakki-Paoli method. Using impedance measurements, a relation between the carrier density, n, and the bias current applied to the laser, I, has been obtained. With this relation, we obtained the differential current with the carrier density, dI/dn. Then, we calculated the differential gain dg/dn = dg/dI * dI/dn. To conclude we saw how the differential gain, dg/dn, has been found to have similar temperature behaviour as the small signal modulation bandwidth.
High power and single mode InGaAsN ridge waveguide lasers were developed. The pulsed the maximum output power was 240 mW at room temperature (RT). The threshold was 15 mA at 20°C. The ridge waveguide laser could work beyond 120°C. For cw operation, the lasers show a maximum output up to 40 mW RT. The broad area lasers using the same materials has been working under continuous-wave operation at constant current (80% of maximum output) for more than 42,800 device-hours at 30°C with as-cleaved facets. They are still working well.
Ultrafast lasers can be used to produce laser pulses with enormous peak powers and power densities. The very high peak power that can be achieved with femtosecond pulses means that in principle, nonlinear frequency conversion should be very efficient. It should be quite straightforward to use second-harmonic (SHG), third-harmonic (THG) and fourth-harmonic generation (FHG) to produce femtosecond pulses in the near- to deep-ultraviolet. We present results on a mode-locked Yb<sup>3+</sup>-fiber laser operating in the 980 nm spectral band. Such lasers are very attractive as a seed source for generating blue light using SHG. The laser comprised a linear fiber cavity defined by the fiber loop-mirror and the semiconductor saturable-absorber mirror (SESAM) used to self-start the mode-locking. SESAM operating in the 940-1050 nm wavelength-range comprised 26 pairs of AlAs/GaAs quarter-wave layers that form a distributed Bragg reflector with a center wavelength at about 1000 nm. The active region consists of five GaInNAs quantum wells embedded within GaAs layers. With proper alignment of the laser cavity, the laser was self-starting for pump powers above 50 mW at 915 nm. The output mode-locked pulse train at about 980 nm had an output power of 3 mW, a repetition rate of 30 MHz and pulse duration of 2.3 ps. The pulse spectrum exhibited soliton sidebands at all pump powers, confirming that the laser operates in the anomalous-dispersion regime. The time-bandwidth product was equal to 0.47, indicating that the pulses were nearly bandwidth-limited with Gaussian temporal and spectral profiles. The average value of the cavity dispersion near 1 µm, estimated from the soliton sidebands, was -1.6 ps<sup>2</sup>. With a master oscillator power amplifier configuration (MOPA) more than 200 mW of the output power is expected with just two single-mode pump laser diodes.
GaInNAs quantum well lasers have attracted significant interest in recent years. Their potential for operation at high temperatures without coolers and their application for low cost vertical-cavity surface-emitting lasers (VCSELs) are the main reasons for this interest. The main consequence of adding Nitrogen (N) to InGaAs materials is the band gap shrinkage. The reason for that is the interaction of N (acting as a localized defect) with the conduction band of the InGaAs. In previous studies, low temperature PL measurements of the impact of Nitrogen on the band structure of GaIn NAs have been examined. Pulsed measurements using a broad area GaInNAs QW laser were carried out and the results were analyzed in terms of the interaction of the N defect state with the GaInAs conduction band edge (band-anticrossing model). A detailed experimental temperature study of single quantum-well GaInNAs lasers at room temperature and above has been carried out. Experimental results of L-I, T<sub>0</sub>, temperature dependence of lasing wavelength, optical gain and efficiencies are presented, discussed and compared with other materials. The temperature ranges studied is appropriate for most network applications. The gain spectra for moderate densities were experimentally measured using the method of Hakki and Paoli: the 600 μm long devise is biased below threshold and the gain is evaluated form the Fabry Perot modulation of the spontaneous emission spectra. A new concept will be introduced to study the bandwidth of the spectral gain and see its dependence with the temperature. The half-peak-BW will be the bandwidth where the gain decreases 50% from the peak gain. The temperature performance of the half-peak-BW has been studied obtaining a slope of 0.5871 nm/K. About the temperature dependence of the laser, a value of T<sub>o</sub> (50 K) similar than the one found in InGaAsP has been found. This might disagree with the first results published of this new material system, giving extremely high values above 100 K. This is due to the high A parameter found in the previous materials. The improvement of the material is decreasing the A parameter and the characteristic temperature of the device. A small temperature dependence of the lasing wavelength was found (0.37 nm/K). This value was confirmed measuring the temperature dependence of the gain peak wavelength. This small temperature dependence can be understood by the interaction of the N state with the conduction band edge.
Before processing the InGaAsN/GaAs edge emitting lasers, post-growth rapid thermal annealing (RTA) was applied on the wafer. Different RTA results in different threshold current density (J<sub>th</sub>). RTA at 720°C reduces the J<sub>th</sub> significantly but keeps the linear fit slope of J<sub>th</sub> vs 1/L (L is the cavity length). It indicates that RTA at 720°C can decrease the absorption losses. High temperature RTA at 890°C can dramatically decrease the linear fit slope, which indicates that the carrier conductivity is improved dramatically even the RTA time is only one second.
We report on the growth of GaInNAs materials and lasers by molecular beam epitaxy (MBE) using a rf-plasma source. Optimal GaInNAs quantum well (QW) structures have been designed and grown in order to achieve the brightest and narrowest photoluminescence (PL) spectra beyond 1.30 um. State-of-the-art GaInNAs/GaAs SQW lasers operating at 1.32 um have been demonstrated. For a broad area oxide stripe, uncoated Fabry-Perot laser with a cavity length of 1600 um, the threshold current density is 546 A/cm2 at room temperature. Optical output up to 40 mW per facet under continuous wave operation was achieved for these uncoated lasers at room temperature.
We report the growth of GaInAsN heterostructures on GaAs substrates by conventional molecular beam epitaxy (MBE) using a radio frequency plasma source. Lattice-matched bulk samples and several strained single quantum well (SQW) and multiple quantum well (MQW) structures were grown. The QWs were sandwiched between two GaAsN strain-compensating layers (SCL) and AlGaAs cladding layers. By the aid of SCLs the photoluminescence (PL) wavelength red-shifted as much as 88 nm with the same intensity. GaInAsN strain-mediating layers (SML), having less strain than QW, were also used to obtain red shift and improved luminescence properties. The structures were studied by room temperature (RT) PL, x-ray diffraction (XRD) measurements and atomic force microscopy (AFM). The indium and nitrogen compositions of the QWs varied from 34 to 38 % and 1.3 to 3.5 %, respectively. Most of the studied structures showed PL peak wavelength at over 1.3 mm. Depending on the structure and thermal annealing treatment conditions the wavelength blue shifted up to 55 nm and intensity increased ~45 times. Furthermore, an AFM image of a five QW sample showed very smooth surface indicating together with PL measurements that high quality MQWs can be realized. In addition, 1.32-micrometers continuous-wave GaInAsN edge-emitting lasers were demonstrated.
We studied the possibilities of LTCC (low temperature cofired ceramics) technology to fabricate transmitter arrays equipped with vertical-mounted multimode fiber pigtails. The developed LTCC module can be mounted vertically on a printed-circuit-board (PCB), thus providing small, essentially one-dimensional PCB footprint. The fiber is aligned and supported using a hole structure through the layers, and the surface-emitting source, such as a VCSEL, is flip-chipped on the other side of the substrate. Thus, this kind of module can be used as a detachable electrical interface between the fiber-optic and electronic media. To evaluate the feasibility of the system, a 5-channel transmitter based on a 4-layer LTCC substrate was designed and realized, and mounted vertically on a test board. Each transmitter, sized 5 mm X 5 mm<SUP>2</SUP>, included VCSEL and laser-driver chips as well as discrete passives. 62.5/125-micron fibers were used with metallic tubes as strain relieves. Before implementation, the optical alignment tolerances were examined by measurements and simulations, and the tolerances of fiber-mounting holes were evaluated by preparing test structures. The fiber alignment accuracy seemed adequate even for the first transmitter prototypes. Nevertheless, stringent requirements for the LTCC process control are necessary to achieve the needed accuracy.