Continuous wave (CW) operation of quantum cascade lasers is reported up to a temperature of 312 K. The junction down mounted devices were designed as buried heterostructure lasers with high-reflection coatings on both facets. This resulted in CW operation at an emission wavelength of 9.1μm with an optical power ranging from 17 mW at 293 K to 3 mW at 312 K. A distributed feedback type device was fabricated and tested as well. It showed CW singlemode operation up to 260 K. These results demonstrate the potential of quantum cascade lasers as CW mid-infrared light sources for high-resolution spectroscopy and free space telecommunication systems.

The work of the authors' group on monolithic integrated in-plane semiconductor lasers using grating components are reviewed and the recent development is reported. The grating components provide not only feedback for lasing but also novel functions such as output beam shaping and wavelength tuning. The design and fabrication of the grating components in semiconductor waveguide are outlined, and the area-selective quantum-well disordering by impurity-free vacancy diffusion is described as an effective technique to reduce the absorption loss in the passive waveguide. Then, device description, design, fabrication and experimental results of integrated master oscillator power amplifier (MOPA) lasers, high-power tunable extended-cavity lasers, and a broad-area angled-grating distributed Bragg reflector (DBR) lasers using InGaAs/AlGaAs GRIN-SCH-SQW structures are presented. All the lasers have integrated beam forming grating coupler, and allow implementation of compact and stable lensless modules that emit a collimated output beam.

We report results on single-mode, InAlGaAs/AlGaAs/GaAs, 915 nm, lser-diodes operating reliably at 300 mW. The graded-index, separate-confinement, strained, single quantum-well structure was grown by metal-organic chemical-vapor deposition. Carbon, rather than zinc, was used as the p-doping srouce to reduce internal loss and potential reliability issues due to the thermal diffusion of zinc. A threshold current density of 133 A/cm², internal loss of 2.0 cm^-1 and internal quatnum efficiency of 93% were achieved. FOr 1500 μm long ridge waveguide lasers, a record single-mode output-power of 500mW was obtained for devices mounted epitaxial-side up onto AlN submounts using eutectic Au₈₀Sn₂₀ solder. Ten burned-in devices have now been aged at a constant current of 450 mA at 85°C for more than 1500 hours wihtout measurable degradation.

Tensile strained GaAsP quantum wells embedded in AlGaAs waveguide structures are used to realize high power, high brightness short wavelength tapered laser diodes. At 735nm these laser diodes show up to 3W nearly diffraction limited output power with a wall plug efficiency of about 40%. Single spectral mode behavior is observed at output power levels up to 1W. From aging test a high realiability with lifetime exceeding 5000 can be derived comparable to results obtained from broad area laser diodes with the same aperture width. There are only small changes of the beam quality during aging. In conclusion it is shown that well designed tapered laser are a step forward to high efficiency, diffraction limited light soruces in the Watt-range which can be easily fabricated in high volumes.

High performance strained-layer InGaAs (λ=1.2μm) and InGaAsN (λ=1.3μm) quantum-well lasers have been realized by MOCVD growth using Arsine and Dimethylhydrazine as the group V precursors. The use of GaAsP high bandgap barrier layers is shown to improve device performance over conventional GaAs barrier lasers. By comparison to conventional InP-based technology, InGaAsN lasers exhibit very low threshold current density at high temperature (390A/cm² at 80°C), using a single-quantum well design.

Dilute nitride GaInNAs alloys grown on GaAs have quickly become excellent candidate materials for lower cost 1.3μm vertical cavity surface emitting lasers (VCSELs) and high power edge emitting lasers. Despite the relative immaturity and challenges of this new materials system the results have been very promising. The major remaining challenges include the limited solubility of nitrogen in GaAs, non-radiative defects that may be caused by nitrogen incorporation, and phase segregation at high In compositions. A new component, Sb, has been added in order to improve epitaxial growth and optical properties at wavelengths longer than 1.3μm. By adding Sb to the alloy, luminescence has been greatly enhanced between 1.3-1.6μm where normally poor quality material results. Lasers fabricated from this new pentenary alloy have produced the lowest threshold lasers operating beyond 1.4μm on GaAs. Progress and the remaining challenges of this materials system are described.

Widely-tunable sampled-grating distributed Bragg reflector (SG-DBR) lasers with integrated Semiconductor Optical Amplifiers (SOAs) simultaneously exhibit high (20 mW CW) fiber-coupled output power, high side mode suppression ratio, low noise (below -140 dB/Hz RIN), low line-width (<5 MHz), and high reliability, across a 40 nm C-band tuning range.

Widely tunable monolithic InP lasers can, in principle, cover one or other of the Er-doped fibre amplifier windows. These windows span wavelength ranges of around 40-50nm. However, the change in refractive index that can be achieved by current injection into a grating section is limited to about 1-2% corresponding to around 10-20nm in wavelength, so some further mechanism is required to extend the tuning range. In this paper, we present a new multi-section, digital supermode DBR laser (DS-DBR) that can be controlled in a simple, quasi-digital manner. The wavelength is coarsely selected by applying current to one of the front contacts to form an enhanced reflection peak and select a sub-band of the total tuning range. Current applied to the rear grating contact allows tuning within that range and a phase section allows fine tuning control. By selecting front contacts in turn, the full tuning range of the device can be accessed. We will present an overview of the device together with some simple modelling to show how the device will perform. This will be followed by a brief description of the fabrication and a comprehensive set of experimental results including RIN and linewidth measurements.

We report on ultraviolet (UV) InGaN and GaN multiple quantum well (MQW) laser diodes grown on c-plane sapphire substrates by metal organic chemical vapor deposition. By reducing the indium content in the InGaN/InAlGaN MQW, we have systematically pushed the room-temperature laser emission to a record low wavelength of 363.2nm. Pulsed threshold current densities around 5 kA/cm² have been achieved for laser diodes with emission wavelength between 368 nm and 378 nm. Light output powers greater than 400mW under pulsed current-injection conditions (pulse duration 500 ns, repetition frequency 1 kHz) and differential quantum efficiencies of 4.8% have been achieved. We also demonstrate room-temperature continuous-wave operation of ridge-waveguide devices with threshold currents of 85 mA for an emission wavelength of 377.8 nm and output power of more than 3 mW.

GaN-based laser diodes (LDs), which emit from UV to blue-green, are reviewed. For fabricating the UV LDs, we used the AlInGaN active layer instead of InGaN one. We demonstrated the UV LDs with a lasing wavelength 368nm under continuous-wave (cw) operation. Moreover, we fabricated the blue-green LDs whose lasing wavelength was 480 nm. It was investigated that the relationship between the threshold current density and the lasing wavelength. From our experiments, we successfully expanded the range of GaN-based LDs lasing wavelength, from UV (368 nm) to blue-green (480 nm). Regarding high power 405 nm-LDs, we could demonstrate the cw operated LD array devices with an output power of 1W by decreasing the thermal resistance of the LD chips.

This paper describes an improved laser structure for AlGaInN based blue-violet lasers (BV-LDs). The design realizes a small beam divergence angle perpendicular to the junction plane and high characteristic temperature wihtout significant increase in threshold current density (J_th) by optimizing the position of the Mg-doped layer and introducing an undoped AlGaN layer between the active layer and the Mg-doped electron-blocking layer. The mean time to failure (MTTF) of devices based on this design was found to be closely related to the dislocation density of ELO-GaN basal layer. Under 50 mW CW operation at 70°C, a MTTF of over 5000 h was realized whenthe dark spot density (indicative of dislocation density) is less than ~5×10⁶ cm^-2. Power consumption under 50mW CW operation at 70°C was approximately 0.33 W, independent of the dislocation density.

The terahertz region (1-10 THz) of the electromagnetic spectrum offers ample opportunities in spectroscopy, free space communications, remote sensing and medical imaging. Yet, the use of THz radiation in all these fields has been hampered by the lack of appropriate, convenient sources. We here report on unipolar semiconductor injection lasers that emit at THz frequencies (4.3 THz, λ ~ 69μm and 3.5 THz, λ ~ 85μm) and possess the potential for device-like implementation. They are based on the quantum cascade scheme employing interminiband transitions in the technologically mature AlGaAs/GaAs material system and feature a novel kind of waveguide loosely relying on the surface plasmon concpt. Continuous-wave laser emission is achieved with low thresholds of a few hundred A/cm² up to 45 K heat sink temperature and maximum output powers of more than 4mW. Under pulsed excitation, peak output powers of 4.5mW at low temperatures and still 1 mW at 65 K are measured. The amximum operating temperature is 67 K.

We review our recent work on lasing in active random media. Light scattering, which had been regarded detrimental to lasing action for a long time, actually provided coherent feedback for lasing. We also trapped laser light in micrometer-sized random media. The trapping was caused by disorder-induced scattering and interference. This nontraditional way of light confinement has important application to microlasers. The threshold of random laser can be reduced by incorporating some degree of order into an active random medium. Our calculation result shows that by optimizing the degree of order one can dramatically reduce the threshold of random laser to the values comparable to htose of photonic bandgap defect lasers.

A broadly tunable range of 200 nm is achieved in external cavity semiconductor laser using a Fabry-Perot laser diode as an amplifier. The broadband tuning is possible due to two reasons. First, the gain bandwidth is broadened using proper design of nonidentical quantum-well structure. Second, carriers distribute over a broad bandwidth, leading to the reduction of gain over a narrow bandwidth. Thus self-oscillation of uncoated Fabry-Perot laser didoes is suppressed, but there is still gain for external-cavity configuration. A tuning range covering from 1340nm to 1540nm is then achieved.

650nm emitting lasers are required for a number of applications including DVD. A combination of requirements, such as narrow far-field divergence, low temperature dependence of threshold current and low absolute operating current make device design difficult. We have adopted the approach of using coupled multiple quantum wells to minimise quasi-Fermi level separation and hence carrier leakage while using a simple low optical confinement structure, with small refractive index step between waveguide core and cladding, to optimise far-field divergence. The samples studied consist of either 3, 4 or 5 coupled quantum wells set in a waveguide core of (Al_xGa_1-x)_0.5In0.5P (0.520.7Ga_0.3)_0.5In_0.5P. The lasing wavelength was between 650 and 655nm with a threshold current density of between 1267Acm^-2 and 1417Acm^-2 for a 450μm long device with a maximum T₀ of 105K (20-70°C). The measured far-field divergence of 27 ±1 degrees (FWHM) was similar for all three structures. We have performed an in-depth analysis using a single pass, multi-section method to deduce modal gain and loss, spontaneous current and efficiency. This demonstrates that the extra wells provide a smaller than expected improvement in gain at room temperature and negligible improvement in gain-current performance at 220K and we attribute this to imperfect carrier distribution amongst the wells. The measured values of the internal optical mode loss of 9-20cm^-1 and approximately 50% for the internal efficiency provide room for improvement in laser performance.

Synchronized external-cavity diode lasers are used for chaotic optical communication. A 1 GHz message is encoded by current modulation of the chaotic transmitter laser and decoded by a synchronzied chaotic receiver laser diode. The effect of introducing the message by optical injection into the receiver laser instead of direct current modulation is then investigated. Finally, we show how the signal masking can be enhacned by the appropriate choice of external cavity length for a given message frequency.

In this paper we present measured modulation responses on quantum-cascade lasers (QCL) up to 10 GHz. The obtained modulation response shows a flat response over the whole frequency range, proving the broadband capabilities of these devices. Even more striking is the absence of a strong resonance peak, which demonstrates the absence of relaxation oscillations, a feature which often limits the high speed performance of standard diode lasers. This property is quite attractive for use of these devices as high-speed data sources, particularly in applications where linearity is important. To address this possible application, the digital modulation properties of these devices were tested in a standard bit-error-rate-measurement at 2.5 GBit/s for cryogenically cooled QCL, showing the suitability of QCLs in digital as well as analog telecommunication application. In addition we present recent data showing bit error measurements and eye diagrams obtained for the pulsed mode operation of QCL near room-temperature.

Two double-waveguide 940 nm asymmetric structures with 8 nm In_xGa_1-xAs quantum well (x = 0.13) are presented. They consist of an active region waveguide that includes a QW and an optical trap waveguide on the n side of the active region. The second structure has a supplementary optical wall in the p - clad that further pushes the optical field from the active region and reduces the confinement factor. The nominal lengths were chosen as 2.8 mm and 4 mm and the design values for the d/gamma ratio were 0.8 μm and 1.14 μm, respectively. Devices were fabricated with 100 μm apertures and yielded threshold current densities of 0.2 kA/cm² and 0.17 kA/cm² with corresponding slope efficiencies of 0.76 W/A and 0.85 W/A. At high current levels devices presented a power saturation behavior. The highest power values were 9.7 W (rollover) and 11.5 W (CMOD), respectively.

High bit-rate, WDM, networks are reliant on Er or Er/Yb doped fiber amplifiers. Reliable, high power laser diodes at 980nm and 1480nm are key devices for pumping these amplifiers. We have developed several 980 nm laser diode structures at 980 nm, using an Aluminum free active region and standard AR/HR coatings on the facets. Our laser show low optical losses, low threshold current density and a high external differential efficiency. We demonstrate a mini-bar of small angle index guided tapered laser diodes (emissive width of 3 mm) with an optical output power of 20W at 33A under CW operation (25°C). The far field of the slow axis has a Gaussian single lobed shape, with a FWHM of 3.5° at maximum power, which is two times less than obtained with multimode broad area lasers. With such a device, we expect to couple 10W into a 100μm diameter fiber. We also demonstrate a large aperature gain-guided tapered laser with an output power of 2.4W and a calculated M²_1/c2 = 3, the M²_1/c2 factor being calculated with the method based on measurements of the fields profiles widths at 1/c².

While high-powered broad area lasers emitting between 915nm and 975nm are required for pumping Er⁺ and Yb⁺ doped dual clad fiber lasers and amplifiers, the single mode 980nm lasers are used for pumping EDFAs. We report on the performance and a systematic reliability assessment of Alfalight’s first generation Al-free multimode laser diodes with 100µm aperture and 2mm cavity length emitting between 950nm and 980nm. Data from 120 devices in five different multi-cell conditions show median life due to wear-out failure to be over 75.5 years. In addition, over 1,307,600 device-hours of accelerated lifetest data at 3A and a 70C heatsink temperature have been accumulated demonstrating 55 FIT (60% confidence level) at a 2W and 25C operation condition. We also present results from a packaged multimode diode laser with wavelength stabilized at 972nm with a spectral FWHM of 0.3nm demonstrating the capability to use such a device for pumping Er⁺ and Yb⁺ doped fibers near the more efficient 975nm portion of the absorption spectrum. Advances made in anti-resonant reflective optical waveguide (ARROW) type single mode diode lasers and the advantages over the conventional positive index guided ridge waveguide type lasers will be discussed. Single mode operation of ARROW single mode laser up to 450mW (ex-facet) was achieved. Results from the facet passivation studies showing successful implementation of non-absorbing mirror (NAM) due to quantum well intermixing using Si implantation in Al-free diode lasers will also be discussed. We have demonstrated reliable operation in excess of 5500 hours in index-guided Al-free diode lasers at a constant power of 500mW at a heatsink temperature of 25C.

Directly modulated External Cavity Laser Diode (ECLD) is an attractive alternative for extended reach source lasers for metro area telecommunications. This paper reports ECLD with transmission rate of 2.5Gbit/s and transmission distance of 650 km packaged in industry standard 14pin butterfly package. High stability package was the key in achieving Telcordia qualification. Combination of narrow linewidth and high bandwidth allows OC-48 transmission for distances comparable with the ones typical for EML. This lasers exhibit +/-30 pm wavelength stability over temperature range of -15 to +75C which makes it suitable for 100 and 50 GHz spacing applications without wavelength locker. ECLD can be operated using standard laser driver. Results using Vitesse 7940 and Analog Devises 2841 laser driver are presented. Typical extinction ratio obtained with these drivers was 10dB. Peak to peak jitter was <40ps. Dispersion penalty with these drivers transmission was found to be <1.9dB for the 650km of standard SMF-28 fiber (No dispersion compensation). Dispersion penalty vs transmission distance data will be presented. Results of the study of nonlinear effects in multi channels link will be reported. We observed that there was self phase modulation effect leading to reduction in dispersion power penalty. This effect can be used to transmit single channel over distances longer then 650km. We showed that there was negligible crosstalk penalty and no FWM penalty. Threshold for stimulated Brillouin scattering was also investigated.