Channeled Substrate Planar Research Articles

External-cavity diode lasers with different devices and collimating optics

Comparative operating characteristics of external-cavity diode lasers (ECDL's) with either a channel substrate planar device or a multi-quantum-well (MQW) device are presented. These include the output beam profile, which is significantly altered depending on the collimating lens used (either multielement or graded index), power versus injection-current characteristics, and the optical frequency and the rf spectra. The coherence lengths of the different laser diode-collimating-lens combinations in the ECDL are measured, and a new method for calculating the coupling coefficient and the coupled values of the internal quantum efficiency and the internal lumped loss is demonstrated for the MQW device.

Structure-dependence of the chirp-to-power ratio spectrum for GaAs external cavity lasers

The chirp-to-power ratio (CPR) spectrum has been modeled for GaAs quantum well external cavity lasers. The model includes effects of transverse carrier spatial hole burning, finite carrier transport and capture time from the separate confinement-heterostructure (SCH) region to the quantum well region, and intrinsic material gain compression. The model explains the measured difference of the phase of the CPR at low modulation frequencies between GaAs quantum well and channel-substrate planar (CSP) lasers both in extended cavities. Our results indicate that the carrier effect in the SCH region can make a major contribution to the CPR. >

Simple method for measuring the linewidth enhancement factor of semiconductor lasers by optical injection locking

A simple method for measuring the linewidth enhancement factor alpha of a semiconductor laser by optical injection locking is described. Without knowledge of the absolute values of detuning and the optical injection level, the value of alpha is evaluated from only changes in optical power in the stable optical injection-locking state. The value of alpha of a 0.83-microm channeled substrate planar laser is evaluated to be 2.65 +/- 0.2. The measurement error of this method is also discussed.

Leaky-guided channeled substrate planar (LCSP) laser with reduced substrate radiation and heating

A semiconductor laser design, the leaky-guided channeled substrate planar (LCSP) structure, is presented. The structure with a buffer layer in the region outside the channel provides a built-in negative effective index step. The transverse mode is well damped in the buffer layer, and the substrate radiation in the region outside the channel is virtually eliminated. As a result, the significant heating and facet degradation in the region outside the channel that is related to the substrate radiation in conventional CSP lasers is minimized. The calculated local temperature rise in the region outside the channel of the CSP laser is about 45 degrees C for an output power level of 100 mW and is in reasonably good agreement with previously experimental results. Analysis using the complex domain effective index method shows that the LCSP structure provides better lateral mode discrimination than either gain-guided or positive index step structures. The LCSP structure with reduced heating and strong lateral coupling effect may be useful for high-power linear array lasers. >

Lateral optical confinement of channeled-substrate-planar lasers with GaAs/AlGaAs substrates

A physical explanation of the lateral guiding mechanism in channeled-substrate-planar (CSP) lasers based on the amount of wavefront tilt of the transverse field outside the channel region is presented. Because of this inherent wavefront tilt, all CSP lasers will have a very slight asymmetry in their transverse far-field pattern. The nature of the guiding mechanism does not require light absorption by the substrate. Design curves showing the complex lateral effective index step as a function of n-clad thickness with the active layer as a parameter are also presented. Depending on the specific layer compositions and thicknesses, the CSP guiding mechanism can provide a positive lateral index step for substrates with mole fractions of AlAs ranging from 0 to higher than 0.2.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Two-dimensional simulation of laser diodes in the steady state

A fully self-consistent steady-state two dimensional model of laser diodes is presented. The model consists of the simultaneous solution of the Poisson and the electron and hole drift-diffusion equations, the wave equation, and the photon rate equation. Excellent agreement with experiment is obtained for both gain-guided and index-guided laser diodes. Specific results are given for channeled-substrate planar (CSP) lasers. It is shown that comparable electron current confinement is provided by both internal strips (p-GaAs barriers) and zinc-diffused planar stripes. The confinement in the first case is due to energy barriers and in the latter case is due to lateral electric fields. For the holes, current spreading is shown to be reduced substantially for the planar stripes because of the use of high-resistivity n-cap layers. It is demonstrated that the thickness of the p-GaAs layers can be smaller than the minority carrier diffusion length since only a very small fraction of the laser light passes through the barriers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of carrier nonuniformity on the phase relationship between frequency and intensity modulation in semiconductor lasers

The modulus and phase of the chirp to modulated power ratio (CPR) was measured for a 1.5- mu m buried-heterostructure laser and a 0.85- mu m channeled-substrate planar (CSP) laser. The results for the phase are inconsistent with previously published expressions including spontaneous emission and spectral hole burning. In particular, the CSP laser exhibits an abrupt phase shift in the CPR. An explanation of this behavior is presented in terms of the influence of a nonuniform carrier density on the phase-amplitude coupling, as expressed by an integral expression for the mode parameter alpha representing the linewidth enhancement factor. Using a simple model for lateral behavior which analytically incorporates diffusion and a nonuniform material alpha parameter, qualitative agreement was obtained with the CSO data. The authors demonstrate the importance of the lateral laser structure on the phase-amplitude coupling in index-guided semiconductor lasers, and the usefulness of CPR phase measurements for laser characterization.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Self-consistent analysis of gain saturation in channeled-substrate-planar double-heterojunction lasers

A self-consistent model for semiconductor lasers [using the channeled-substrate-planar (CSP) double-heterojunction (DH) laser as an example] which does not assume constant optical power along the laser axis is developed. This approach allows for the analysis of high-power lasers with low facet reflectivities which produce nonuniform photon densities along the propagation direction. Analytical equations for the modal gain coefficient, the threshold current density, and the radiated power for a specific CSP laser structure are obtained.

Observations and consequences of nonuniform aluminum concentrations in the channel regions of AlGaAs channeled-substrate-planar lasers

Compositional changes in the n-clad layer within the channel region of channel substrate planar (CSP) type semiconductor lasers have been observed. As a consequence, a large optical cavity (LOC) or an enhanced substrate loss (ESL) version of the CSP geometry may result, both of which may have significantly different characteristics from those of a conventional CSP laser. The CSP-LOC generally has a larger near-field spot size, while the ESL-CSP is characterized by an off-axis, asymmetric far-field pattern.

Rigorous waveguiding analysis of the separated multiclad-layer stripe-geometry laser

The waveguide modes of separated multiclad-layer (SML) laser diodes are calculated accurately by means of laterally-coupled mode equations. Thereby, it is shown that the mode losses are appreciably higher as compared to previous calculations. In particular, enhancement of the mode losses in narrow-stripe SML lasers ( w \leq 5 \mu m) may explain the relatively high threshold currents and low efficiencies reported so far for AlGaAs-GaAs and InGaAsP-InP SML lasers. A detailed investigation of various geometries reveals this laser structure to equal the related channeled substrate planar (CSP) laser with respect to the waveguide losses.

Mode characteristics of the multiple-stripe laser effects of the loss embedded in the outer unpumped region

The criteria for the mode selections in multiple-stripe lasers are presented. It is shown that the couplings of the higher-order modes of each stripe, which constitute the multiple-stripe laser, play important roles in the mode selections. Based on this analysis, a method for realizing 0° phase-shift mode oscillations is proposed. The method involves loss embedment in the outer unpumped region and the use of multiple stripes with fine center-to-center spacings. For the realization of this scheme, a grating-incorporated channeled-substrate planar (GICSP) laser is proposed and is fabricated. It is demonstrated that the 180° phase-shift mode is completely suppressed with this scheme. Single-longitudinal mode oscillations of well-defined index-guided 0° modes are observed for wide-channel CSP lasers.

High-power single longitudinal mode operation of inverted channel substrate planar lasers

A high-power GaAs/GaAlAs double heterostructure laser is developed by using the metalorganic chemical vapor deposition technique. The laser has an index-guided, inverted channel substrate planar structure. Linear output power characteristics were observed to ∼80 mW cw on submount without facet coatings. The device operated in a stable single mode to a power &amp;gt;40 mW.

Theory of transverse multimode generation in a laser with a loss guiding structure

A theoretical consideration is given of the transverse multimode generation in a laser which is equipped with a lossy layer below an active layer for mode control, e.g., the buried twin-ridge substrate laser, the channeled substrate planar laser, and the V-shape inner stripe laser. The multimode generation occurs as a result of the reduction of the first-order mode threshold to the fundamental mode threshold. It is shown that even a small asymmetry of the lossy layer causes large reduction of the first-order mode threshold.

Mode characteristics of grating-incorporated channeled-substrate-planar GaAlAs lasers

A grating-incorporated channeled-substrate-planar (CSP) laser is proposed for the 0° phase-shift mode selection. The 180° phase-shift mode is suppressed by combining fine gratings with the loss outside the electrode region. Single longitudinal mode oscillations of well-defined index-guided 0° modes are demonstrated for the wide channel CSP lasers.

High-power operation of index-guided inverted channel substrate planar (ICSP) lasers

An easily fabricated high-power index-guided laser, the inverted channel substrate planar (ICSP) structure, has been developed using the two-step metal-organic chemical vapour deposition (MOCVD) technique. Linear output power characteristics were observed to 30 mW CW on submount with junction side up without facet coatings. The device operated in a stable single mode to a power >25 mW. Improved ICSP lasers with facet coatings demonstrate a stable high output power of 65 mW CW and a differential quantum efficiency of 60%.

Coupled-mode analysis of gain and wavelength oscillation characteristics of diode laser phased arrays

The lasing wavelengths and gain characteristics of the modes of phase-locked arrays of channel-substrate-planar (CSP) lasers are presented. The gain values for the array modes are determined from complex coupling coefficients calculated using the fields of neighboring elements of the array. The computations show that for index guided lasers which have nearly planar phase fronts, the highest order array mode will be preferred. The "in phase" or fundamental mode which produces only one major lobe in the far-field radiation pattern, has the lowest modal gain of all array modes. The modal gain differential between the highest order and fundamental modes is less than 10 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for weak coupling between the elements.

Magnitude and phase characteristics of frequency modulation in directly modulated GaAlAs semiconductor diode lasers

The magnitude and phase of the small-signal injection current-to-frequency modulation transfer function in GaAlAs semiconductor diode lasers has been measured over the frequency range 100 Hz-1300 MHz using network analyzers. Channeled substrate planar (CSP), buried heterostructure (BH), and crank transverse junction stripe (TJS) laser structures were investigated and will be compared. Approximately 180, 180, and 90° phase differences between the low frequency, thermal frequency modulation (FM), and the high-frequency carrier-density FM was observed for BH, TJS, and CSP laser structures, respectively. The origin of this phase difference and its implications for FM optical communications will be discussed, A rate equation analysis for small-signal injection current modulation (using Mason's flowgraph representation) indicates the presence of a real-axis left half-plane zero in the carrier-density small-signal frequency modulation response. Experimental evidence for this feature has been observed in the measured injection current-to-FM transfer functions.

100-Mbit/s semiduplex optical fiber transmission experiment using GaAs/GaAlAs laser transceivers

Semiduplex, high-data-rate transmission over a single optical fiber using GaAs/GaAlAs channeled substrate planar (CSP) structure lasers both as light source and photodetector has been demonstrated. The responsivity in the receiving mode at the operating wavelength (830 nm) was 0.15 A/W. This device is suitable for semiduplex transmission, since the emissive spot and the photosensitive area of the laser coincide. The impulse response was measured to be 2.5 ns full width at half maximum (FWHM) at zero bias voltage. A 500-m single-mode fiber was used, and by using microlenses the input coupling loss was reduced to less than 4.5 dB. Measurements at a data rate of 100 Mbit/s return to zero (RZ) shows an optical power margin of 14 dB for a bit error rate (BER) of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> . Using lasers instead of LED's as transceivers would improve the transmission capability and the system margin of a semiduplex communication link.

Estimation of the Ultimate Frequency Stability of Semiconductor Lasers

The frequency stabilities of 0.8 µm AlGaAs lasers were estimated by using the Allan variance as a measure of the stability. The contributions of the quantum noise (the spontaneous emission, carrier, and current noise) and additional noise (current source noise and temperature noise) are given. The highest frequency stability of the free-running laser was estimated to be 6.3×10-11 at an integration time of 0.1 s. It is shown that the frequency stability of the stabilized laser is limited by the quantum noise. The estimated results were compared with the experimental results and with the estimated stability of 3.39 µm He–22Ne lasers. The derivations of the spectral width from the frequency stability are also given. The narrowest limit of the spectral width was estimated to be 5.5 MHz (HWHM) for I/Ith=1.3, while the corresponding experimental result was 6.2 MHz for a channeled-substrate planar (CSP)-type laser.

High-performance single-mode AlGaAs Gaussian channel substrate planar laser diodes

Stable fundamental transverse and single longitudinal mode CW behaviour is demonstrated using a Gaussian channel substrate planar waveguide laser. Highly reliable performance including low threshold current, extremely low spontaneous background level and high-power CW operation has been achieved. In addition, the longitudinal mode has a locking range of 7.8°C, where the wavelength changes at a rate of 0.6 Å/°C.