GaN Laser Diodes Research Articles

Facet degradation of GaN heterostructure laser diodes

We investigated the degradation of cleaved facets of (Al,In)GaN laser diodes in different atmospheres. We found that operation in water-free atmospheres with sufficient oxygen shows a slow degradation. Operation in atmospheres with water vapor causes a fast degradation and an oxidation on the facet. This deposition is a permanent damage to the laser diode. If the laser diode is operated in pure nitrogen, we find a thick deposition on the facet, which shows high absorption. This deposition can be removed by either high optical output powers or by operation in atmospheres with sufficient oxygen. We also explain the influence of these coatings to the degradation behavior and see these coatings as the reason for unstable kinks in the L–I characteristics during operation.

Near-field and far-field dynamics of (Al,In)GaN laser diodes

Near- and far-field dynamics of edge-emitting (Al,In)GaN laser diodes are measured simultaneously with a 100 nm spatial and a 5 ns temporal resolution using a scanning near-field microscope. We reconstruct the phase distribution at the laser diode facet. Beam steering and near-field mode dynamics are interpreted in terms of thermal and carrier induced change of refractive index in the waveguide.

Influence of ridge geometry on lateral mode stability of (In/Al)GaN laser diodes

As a contribution to the current intense research efforts on wide band-gap semiconductor devices which are directed towards the development of commercial blue laser diodes, in this issue's Editor's Choice the optimization of the waveguide structure, to achieve stable laser mode operation, is investigated. The cover picture shows the near-field intensity distribution of the fundamental and first order lateral modes of a blue (Al,In)GaN laser diode together with the double longitudinal mode comb associated with these modes. The first author, Ulrich T. Schwarz, works on optical spectroscopy, characterization, and simulation of blue laser diodes and LEDs. Apart from semiconductor optoelectronics, the focus of his scientific interest is on modern optics, covering propagation invariant Bessel beams, Ince-Gaussian beams and optical vortices.

Influence of ridge geometry on lateral mode stability of (Al,In)GaN laser diodes

AbstractHeight and width of the ridge forming the laser diode waveguide determine threshold current density and lateral mode stability. We measure the optical near‐field of the laser mode and simulate the two–dimensional mode distribution including waveguide losses and optical gain. The simulations show that weak guiding and not current spreading is the major cause for increased threshold current densities in weakly guided laser diodes. The near‐field measurements show fundamental and higher order modes for nominally identical ridge laser diodes. We demonstrate that asymmetric losses in the waveguide bias the lateral mode competition towards higher order modes for an intermediate range of index guiding. This asymmetric damping is specific for (Al,In)GaN laser diodes due to absorption introduced in the p‐waveguide by magnesium doping. This competition of lateral modes can also be seen spectrally as two longitudinal mode combs of different optical gain. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Diode pumping of a continuous-wave Pr^3+-doped LiYF_4 laser

We report, for the first time to our knowledge, diode-pumped cw laser oscillation of Pr3+:LiYF4 in the red spectral range. The pump power is provided by a GaN laser diode emitting a maximum output power of 25 mW at a wavelength of approximately 442 nm. The Pr3+ laser emits 1.8 mW of output power at a 639.7-nm wavelength. Threshold pump power and slope efficiency in a nonoptimized setup are determined to be 5.5 mW and 24%, respectively.

Facet degradation of (Al,In)GaN laser diodes

We study the facet degradation behavior of (Al,In)GaN multiple quantum well laser diodes. Water vapor causes a fast degradation due to facet oxidation of the uncoated facet. Degradation in an inert nitrogen atmosphere is slow and comparable to degradation of GaN laser diodes with coated facets. We also observe a reversible increase in the threshold current density due to a change in absorption caused by surface charges. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Gain spectra and current-induced change of refractice index in (In/Al)GaN diode lasers

Adapting the Hakki–Paoli method for blue laser diodes we measure gain spectra for (In/Al)GaN on SiC substrate laser diodes in the low carrier density regime. From the measured longitudinal mode spacing we calculate the effective refractive index and can exclude a mode spacing ‘anomaly’. We demonstrate that the substrate has a considerable influence on the modal gain. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A Compact 209-nm Deep UV cw Light Source for Spectroscopy Using Frequency Doubling of a Diode Laser

We report on the development of a tunable continuous-wave coherent light source near 209 nm using frequency doubling of a GaN diode laser. The second harmonic wave of the 418-nm output of a GaN diode laser was generated in a single β-BaB2O4 (BBO) crystal placed in an enhancement cavity. The diode laser was optically stabilized to the enhancement cavity through feedback from a counterpropagating fundamental wave, which is weakly excited in the resonator. The output power of 9.1 µW at 209 nm has been obtained, and about 11 GHz continuous tuning without mode hopping has been achieved.

Antireflection-coated blue GaN laser diodes in an external cavity and Doppler-free indium absorption spectroscopy

Commercially available GaN-based laser diodes were antireflection coated in our laboratory and operated in an external cavity in a Littrow configuration. A total tuning range of typically 4 nm and an optical output power of up to 30 mW were observed after optimization of the external cavity. The linewidth was measured with a beterodyne technique, and 0.8 MHz at a sweep time of 50 ms was obtained. The mode-hop-free tuning range was more than 50 GHz. We demonstrated the performance of the laser by detecting the saturated absorption spectrum of atomic indium at 410 nm, allowing observation of well-resolved Lamb dips.

Amplified luminescence and threshold current temperature dependencies of ZnSe and GaN laser diodes

Using a transfer equation approach, it is shown that the cavity-trapped amplified luminescence (AL) flux densities S integrated over the frequency and cavity length at the threshold of ZnSe, zinc-blende and wurtzite GaN laser diodes (LDs) with stripe-geometry bulk active layers reach (3.5–9)×104 MW/m2 within the temperature range 200–400 K. At these values of S, the nonlinear optical effects induced by AL can be observed. The AL induced recombination can enhance the threshold current density of the short wavelength LDs by two or even three times.

Frequency stabilization of an extended-cavity violet diode laser by resonant optical feedback

Frequency-stable operation of an extended-cavity GaN diode laser was achieved by resonant optical feedback from a confocal cavity. The observed frequency fluctuations were smaller than 300 kHz peak-to-peak in one second, and the upper limit of the linewidth was measured to be 880 kHz. A continuous frequency scan up to 5 GHz was realized by simultaneous servo control on the optical feedback phase and on the extended-cavity diode laser frequency. The violet laser was used to perform saturation spectroscopy on the 5 2 S 1/2–6 2 P 3/2 transition of rubidium at 420 nm in a gas cell, and resulted in spectra with a linewidth of 2.3 MHz.

Realization of a GaN Laser Diode with Wet Etched Facets

Realization of a GaN Laser Diode with Wet Etched Facets

Realization of a GaN Laser Diode with Wet Etched Facets

Realization of a GaN Laser Diode with Wet Etched Facets

Challenge of Near-Field Recording beyond 50.4 Gbit/in2

The possibility of an areal density over 50 Gbit/in2 was examined in near-field phase-change recording. The disk structure was optimized to maximize readout signals under the land-and-groove recording condition at a tracking pitch of 160 nm. We also evaluated the signal crosstalk from adjacent tracks. Eye diagrams of 50.4 Gbit/in2 areal density were demonstrated using 1.5 NA optics and a GaN laser diode. The track pitch and linear bit density are 160 nm and 80 nm/bit, respectively. The transmission electron microscope (TEM) micrograph of recorded amorphous marks at an areal density of 50.4 Gbit/in2 is also presented.

Optical Properties and Lasing in (In, Al)GaN Structures

We achieved low-threshold ultra-violet lasing in optically pumped GaN/AlGaN separate confinement heterostructures over a wide temperature range. Lasing modes of a single microcavity were examined from 20 to 300 K and gain mechanisms were compared to those of a thick GaN epilayer. We have also systematically studied InGaN/(In)GaN multiple quantum wells as a function of well and barrier thickness. We demonstrate that the stimulated emission threshold and photoluminescence (PL) decay time are strongly dependent on the well and barrier thickness. The experimental results indicate that the enhanced optical quality of samples with larger barrier thicknesses can he readily applied to the fabrication of InGaN/(In)GaN laser diodes.

Autocorrelation of femtosecond pulses from 415–630nm using GaN laser diode

Using the two-photon response of a commercial 39 3nm GaN laser diode the authors have measured intensity and interferometric second-order autocorrelations of visible femtosecond pulses in the 415-630 nm wavelength region for pulse energies as low as 1 pJ.

High Data-Rate Phase-Change Media for the Digital Video Recording System

The digital video recording (DVR) system combines a high numerical aperture (NA=0.85) with a thin cover layer to achieve higher storage capacities in optical recording. In addition to capacity, the data-rate is a key parameter for DVR and a major challenge in research on phase-change materials. Using Ge2Sb2Te5 as the phase-change material, we demonstrate recording using a red laser diode (λ=650 nm) at a user data-rate of 33 Mbps and a capacity of 9.2 GB for a single layer disc of 12 cm diameter, i.e. completely satisfying all the DVR system requirements. In addition, we achieved data-rates of up to 50 Mbps by applying a different stack design. Furthermore, we extend DVR to the blue-violet wavelength region. Using a GaN laser diode (λ=400 nm) in our recording set up, a capacity of 21.5 GB was achieved at a data-rate of 28 Mbps.

Optical Disk Recording Using a GaN Blue-Violet Laser Diode

The combination of a GaN laser diode and a 0.85 numerical aperture objective has achieved an optical storage capacity of over 22 GB. Owing to sufficient modulation ability and low-noise characteristics, GaN semiconductor lasers possess adequate quality to be light sources in optical recording systems. A bit size of 130 nm×300 nm, corresponding to the areal recording density of 16.5 Gbit/in.2, was realized on a six-layered thin-cover phase-change disk with a crystallization-promoting structure. Each active layer was inversely stacked and was thermally as well as optically optimized for the blue-violet light sources. A new small electromagnetic actuator with a lightweight φ3 mm two-element lens has extended its servo-bandwidth up to 8 kHz and enabled precise focusing control at the data transfer rate of 35 Mbps.

Phase-change Optical Disk Technologies for Utilizing Blue-Violet Laser.

The use of blue-violet laser sources such as SHGs devices and GaN laser diodes is one of the most promising strategies for continuing to increase the recording capacity of phase-change optical disks. Shorter wavelength lasers can be combined with other approaches, including the use of an objective lens with large NA of close to or exceeding 1.0 and a dual-layer disk structure. Phase-change materials for use under these conditions are required to show large changes in the optical constants of n or k combined with sufficiently large optical transmittance at the shorter wavelength, as well as a sufficiently high crystallization rate to compensate for the smaller laser spot size. It is thought that Ge-Sb-Te will be applicable to the memory layer ofphase-change optical disks using blue-violet laser light in the same way as red laser light is now used.

High density optical disk system using D8-15 modulation code and new signal-processing techniques

We developed a new modulation code that is effective for advancing a high-density optical disk and a reproduction signal-processing system. The reproduction characteristics of a ROM disk having a recording capacity of 19.8 Giga-bytes (GB) are examined with combining the newly developed reproduction signal-processing system with an optical system composed of a NA=0.7 single objective lens and a GaN laser diode.