Analysis of diode laser properties

An analytic model of diode lasers applicable to both the lasing and the nonlasing states is described. For these homogeneously broadened devices, spectral envelope widths for TE00and TM00modes are related to power in each modal family and are shown to depend critically on spontaneous emission coupling into the transverse modes. Thus, lasers with real-refractive index waveguiding (and associated weak spontaneous emission coupling) operate single longitudinal mode above threshold, whereas gain-guided devices run multimode. After connecting gain and spontaneous emission, a charge conservation equation, containing optical power in the form of a stimulated emission term, pumping current, spontaneous emission, and spectral width, is derived. These equations are then demonstrated to suffice for determination of the completeLversusIcharacteristic. For lasers in which both charge and mode confinement exist, such as the buried heterostructure (BH) and channelled-substrate narrow stripe (CNS) types, it is shown that both TM00power and spectral envelope width approach limiting values at threshold, whereas TE00mode power grows in conjunction with TE00spectral envelope narrowing.     

Far-infrared guided wave optics experiments with anisotropic crystal quartz waveguides

The TE and TM modes of anisotropic dielectric waveguides of crystal quartz have been studied in the far infrared using an optically pumped waveguide laser with a hybrid output mirror. Using Si and Ge prism couplers, excitation efficiencies of 56 percent of the theoretically calculated values were obtained for the TE0mode. The measured propagation constants were found to agree closely with theory when the perturbation of the guided modes by the prism couplers was included. The attenuation constant for the TE0mode atlambda_{0} = 496.1mum was found to be as low asalpha_{TE_{0}} = 0.05cm^-1. A metal grating coupler for backward-wave coupling through a prism shaped TPX plastic substrate was also used to couple 89 percent of the guided energy in the TE0mode out of the waveguide in the reverse direction.     

CW laser operation of Nd:YLF

CW laser operation of Nd:YLF is reported for the first time and, in a comparison to Nd:YAG, exhibits a lower threshold and higher single-mode average power. The TEM00mode volume of Nd: YLF was observed to be a factor of four larger than Nd:YAG, resulting in twice the TEM00mode average power. This result is attributed to the substantially lower thermal lensing of YLF with respect to YAG. In comparative measurements the stimulated emission cross section of Nd:YLF was found to be1.8 times 10^{-19}cm²for the π oscillation and1.2 times 10^{-19}cm²for the σ oscillation, as compared to2.4 times 10^{-19}cm²for Nd:YAG. The spectroscopic and lasing parameters of Nd:YLF indicate a potential for this material in high peak and average powerQ-switched applications.     

Dispersion of the group velocity refractive index in GaAs double heterostructure lasers

The dispersion of the refractive index corresponding to the group velocityn*_{1}has been measured as a function of wavelength. It is obtained from the longitudinal mode spacing of GaAs buried heterostructure lasers at threshold. The dependence ofn*_{1}on wavelength contains an approximately constant term due to the refractive index n1and a strongly dispersive component due to-lambda (partialn_{1}/ partiallambda). For a given spectral bandwidth, the dispersion ofn*_{1}causes a temporal broadening of a pulse as it passes through the medium. This dispersive effect is shown to contribute to the width of 0.65 ps long pulses obtained recently from mode locked semiconductor lasers. By reducing the length of the laser, the dispersive effect is reduced and it is suggested that pulses as short as 10^-13s should be obtainable from such mode locked semiconductor lasers.     

Optical losses and efficiency in GaAs laser diodes

Optical gain and losses of GaAs laser diodes were deduced from the length dependence of the threshold current density and the differential external quantum efficiency at 77 and 300°K. It has been confirmed that the optical gain is proportional to the threshold current density Jt; and the optical losses are given by the sum of current independent termalpha_{0}', and current dependent termbeta"J_{t}, which is proportional to the threshold current density. The termbeta"J_{t}depends on the laser wavelength because the threshold current density depends on the laser wavelength. It is believed thatbeta"J_{t}is caused by the penetration of the laser oscillations into the noninverted regions surrounding the inverted population region of thep-njunction.     

Lateral analysis of quasi-index-guided injection lasers: Transition from gain to index guiding

In double-heterostructure stripe-geometry semiconductor lasers an effective lateral index stepDelta n_{L}over the stripe region can be induced through evanescent-field coupling. Such a quasi-index-guided device exhibits a transition from the gain-guided to the index-guided regime whenDelta n_{L}is progressively increased. Using parameters appropriate to a 1.3-μm InGaAsP laser, the transition is shown to occur aroundDelta n_{L} sim 5 times 10^{-3}. The exact value ofDelta n_{L}depends on the extent of carrier-induced antiguiding. In the transition region the threshold current decreases rapidly, the lateral mode contracts, and the far field changes from a twin-lobe to a single-lobe pattern. Our analysis suggests that a quasi-index-guided device operates most efficiently for values ofDelta n_{L}at which the index-guided regime is just approached. With a further increase ofDelta n_{L}, the mismatch between the gain and mode profiles leads to lower differential quantum efficiencies. Among other structures, the analysis is applicable to a ridge waveguide laser. For a 1.3-μm laser the optimumDelta n_{L}can be obtained using 0.2-μm-thick cladding layers for a 0.2-μm thick active layer.     

A high power gyrotron operating in the TE041mode

A 35-GHz gyrotron operating in the TE041mode has been built and tested. Output powers of 320 kW at 40-percent efficiency have been obtained. These test results agree closely with theoretically predicted results. Regions of stable operation in the TE041mode were also determined experimentally. These regions could be predicted by a small signal analysis of threshold currents.     

Operating characteristics of single-quantum-well AlGaAs/GaAshigh-power lasers

The operating characteristics of six types of graded-index separate confinement heterostructure single-quantum-well wide-stripe lasers grown by metalorganic chemical vapor deposition are reported. The lasers exhibited intrinsic mode losses as low as 3 cm^-1 and internal quantum efficiencies near unity. Measured differential gain coefficients range from 3.7 to 6.5 cm/A, and extrapolated transparency current densities range from 54 to 145 A/cm². These wide-stripe lasers are typically multilongitudinal mode and exhibit narrowing of the gain envelope and lateral far-field pattern as the cavity length increases. The high value of T₀(>200 K) at long cavity lengths in conjunction with the low current density permits junction-side-up operation to CW optical powers of 0.5-0.7 W/facet, at which level catastrophic facet damage occurs on the uncoated devices. A maximum power conversion efficiency of 57% was measured on the laser structure exhibiting the lowest threshold current     

Short-Pulse High-Power Operation of GaSb-Based Diode Lasers

We report on the short-pulse high-power operation of GaSb-based diode lasers emitting in the 1.95- to 2.2-$muhbox{m}$ wavelength range. Both epi-side-down mounted broad-area lasers and epi-side-up mounted ridge waveguide lasers, sharing the same active region and epitaxial layer structure, have been studied. They show drastically different output power characteristics and thermal limitations in continuous wave (CW) or quasi-CW operation ($hbox{long ~current ~pulses}> 1~mu hbox{s}$) due to the vastly different laser geometries and mounting configurations. However, in short-pulse operation ($hbox{pulse ~length} le 50~hbox{ns}$), the maximum output power density was found to be almost independent of the device geometry and the mounting technique, even though still limited by thermal rollover due to self-heating of the device active region. With the heatsink temperature set to $-$30 $^{circ}hbox{C}$, a maximum single-emitter output power of 22.5 W was recorded at an injection current density of 110 $hbox{kA}/hbox{cm}^{2}$ (50-ns pulse length), and a maximum power density at the output facet of 26.7 $hbox{MW}/hbox{cm}^{2}$ (30-ns pulse length) was achieved, without encountering catastrophic optical mirror damage.      

InP-In1-xGaxAsyP1-yembedded mesa stripe lasers

A laser structure was fabricated using two-step liquid-phase epitaxy, employing the melt-back technique. The fabrication and properties of this structure are described in detail. Good linearity of the power output up to power levels of 20 mW was obtained. The threshold current density at 300 K is 9-12 kA/cm². This high value is mainly due to Zn-diffusion from the third to the buffer layer during the second growth step of the fabrication process. The external differential quantum efficiency is 30-35 percent under pulsed operation at 25°C. The pulsed threshold current has an exponential behavior with temperature whereT_{0} = 60degC. The far-field beam divergences in the directions parallel and perpendicular to the junction plane are12-15degand35-40deg, respectively. Transverse mode stabilization was improved with this laser structure.     

AlInP–AlGaInP Quantum-Well Lasers Grown by Molecular Beam Epitaxy

We have examined a possibility to use an Al $_{x}$In $_{1 - {x}}$P layer as an active region of a 650-nm semiconductor laser. Encouraging results have been obtained with compressively strained oxide-stripe AlInP–AlGaInP quantum-well lasers, which operated in continuous-wave mode at room temperature, producing an optical power of 460 and 320 mW per uncoated facet at 10 $^{circ}$C and 20 $^{circ}$C, respectively. In pulsed mode, a power level of 780 mW/facet was achieved at 2-A drive current at 5 $^{circ}$C. The results indicate that wide-bandgap AlInP affords an opportunity to develop lasers for the wavelengths $600leq lambda leq 650$ nm, which is difficult to achieve by any semiconductor heterostructure.      

2 MHz AO $Q$ -switched ${rm TEM}_{00}$ Grazing Incidence Laser With 3 at.% Neodymium Doped Nd:YVO$_{4}$

We present a detailed experimental and theoretical study of the ultrahigh repetition rate AO $Q$ -switched ${rm TEM}_{00}$ grazing incidence laser. Up to 2.1 MHz $Q$-switching with ${rm TEM}_{00}$ output of 8.6 W and 2.2 MHz $Q$ -switching with multimode output of 10 W were achieved by using an acousto-optics $Q$ -switched grazing-incidence laser with optimum grazing-incidence angle and cavity configuration. The crystal was 3 at.% neodymium doped Nd:YVO$_{4}$ slab. The pulse duration at 2 MHz repetition rate was about 31 ns. The instabilities of pulse energy at 2 MHz repetition rate were less than ${pm}6.7hbox{%}$ with ${rm TEM}_{00}$ operation and ${pm}3.3hbox{%}$ with multimode operation respectively. The modeling of high repetition rate $Q$-switched operation is presented based on the rate equation, and with the solution of the modeling, higher pump power, smaller section area of laser mode, and larger stimulated emission cross section of the gain medium are beneficial to the $Q$-switched operation with ultrahigh repetition rate, which is in consistent with the experimental results.      

Asymmetric λ/4-shifted InGaAsP/InP DFB lasers

1.5 μm asymmetriclambda/4-shifted InGaAsP/InP DFB lasers, in which thelambda/4-shift position was moved from the center of the DFB region toward the front side, were made in order to obtain higher output power with high single-longitudinal-mode (SLM) yield. Statistical measurements revealed that it was effective for the increase of the differential quantum efficiency from the front facet without a remarkable decrease of the SLM yield to move thelambda/4-shift position to the front facet by 10-15 percent of the total DFB length. The output efficiencies of the diodes with AR coatings on the window structure were almost coincident to those expected from theoretical calculations. The main to submode ratioP_{0}/P_{1}in the rear spectra of the asymmetric devices noticeably decreased with increasing asymmetry. From an experimental point of view, a criterion for stable SLM operation, for example,P_{0}/P_{1} gsim 1.5in the rear spectrum atI = 0.9 Ithfor thelambda/4-shift positionl_{f} : l_{r} = 35 : 65, is presented.     

Analyses of mode-hopping phenomena in an AlGaAs laser

Intensity fluctuations of the longitudinal modes of a 0.8 μm AlGaAs laser were precisely measured during the occurrence of hopping between two modes. It was found from this result that mode hopping follows the stochastics of a Poisson process. The frequency of mode hopping was measured asf_{c} = [exp [-95(I/I_{th} - 1)]] times 10^{7}(Hz). whereI/I_{th}is the injection current normalized to its threshold value. Results of analog computer simulations showed that spontaneous emission worked as a triggering force for mode hopping. Results of the analysis based on the Fokker-Planck equation were compared to the experimental results, from which the root-mean-square value of the fluctuating electric field of spontaneous emission was estimated as2.3 times 10^{2)(V/m)leqlanglesim{E}_{N} leq 3.2 times 10^{2}(V/m). It is concluded that an effective reduction of mode hopping is achieved if the laser is operated at a higher bias or if the coupling constant between the two modes is increased.     

Precision Threshold Current Measurement for Semiconductor Lasers Based on Relaxation Oscillation Frequency

The soft turn-on of semiconductor lasers leads to uncertainty in defining and measuring the laser threshold injection current, ${I}_{rm th}$. Previously, practical calculation algorithms have been developed to achieve high-accuracy measurement of a clearly defined and reproducible quantity which is called ${I}_{rm th}$. We demonstrate a new and higher accuracy measurement of ${I}_{rm th}$ using the dependency of the relaxation oscillation frequency on injection current, as compared to the existing standardized approaches. Further, if it is accepted that relaxation oscillations do not occur below laser threshold, this may be regarded as a more fundamentally based definition and measurement method to determine the laser threshold injection current in a semiconductor laser. The method may also be applicable to other types of lasers.      

Gain spectra in GaInAsP/InP proton-bombarded stripe-geometry DH lasers

We have measured gain spectra for TE polarization in a GaInAsP/InP laser as a function of dc bias current below laser threshold. The measurements were made on a low-threshold device with a stripe geometry defined by proton bombardment. A number of other characteristics of this and similar devices from the same wafer are also reported. These data permit the maximum gain coefficient of the active layer gmaxto be evaluated as a function of nominal current density Jnom. We obtain the linear relationshipg_{max} = (3.1 times 10^{-2}/eta_{r})(J_{nom} - eta_{r} 5.4 times 10^{3}), where ηris the radiative quantum efficiency. Our data apply only for large gain (g gsim 150cm^-1) and large Jnombecause the active layer of the test device is thin (0.1 μm).     

High-power high-reliability operation of 1.3 µm p-substrate buried crescent laser diodes

Stable high-power CW operation of 1.3-μm InGaAsP/InP p-substrate buried crescent laser diodes (PBC-LD'S) has been realized, by controlling the front and rear facet reflectivities of the laser diode chips. The front facet reflectivity is reduced to 17 percent and the rear facet reflectivity is increased to 90 percent, by evaporating multilayer dielectric films (Si/Al2O3SiO2:17 percent, SiO2/Si/SiO2/Si/SiO2:90 percent) on each facet. CW light output power of 50 mW is achieved up to 60°C. Aging tests have been carried out under automatic power control (APC) mode conditions of 50°C-30 mW, 40 mW, 50 mW, and 30°C-50 mW. All samples are operating stably in spite of junction-up configuration. The lifetimes are estimated to be more than2 times 10^{4}h for all conditions.     

Thin-film tunable optical filtering using anisotropic and noncollinear acoustooptic interaction in LiNbO3waveguides

Theoretical and experimental studies of optical filtering which utilize anisotropic and noncollinear acoustooptic interaction in aY-cut LiNbO3waveguide are reported. A guided-light beam from either an He-Ne laser at 6328 Å or an argon laser at around 5000 Å, propagating at an angle centering around 70° from theZ(c)axis of the LiNbO3crystal, was Bragg diffracted by the surface acoustic wave with 500 MHz center frequency and propagating at 16° from thexaxis of the crystal. High optical resolving power, large optical angular aperture, and good spatial separation between the filtered light and the unfiltered light have been simultaneously achieved. An optical passband of 16 Å and an angular aperture of 9° have been measured for the case in which the undiffracted and diffracted light propagate in TE0mode and TM0mode, respectively. Furthermore, by varying the acoustic frequency from 350 to 670 MHz, the filtered optical wavelength may be tuned from 8300 to 5000 Å.     

Injection-Type GaInAsP–InP–Si Distributed-Feedback Laser Directly Bonded on Silicon-on-Insulator Substrate

An injection-type distributed-feedback laser, with wirelike active regions, directly bonded on a silicon-on-insulator substrate, was realized. A low threshold current $I _{rm th}$ of 104 mA was obtained at a stripe width of 25 $mu$m and a cavity length of 1 mm. A sidemode suppression ratio of 28 dB was obtained at $1.3 I _{rm th}$.      

Transverse mode controlled in inGaAsP inP lasers at 1.5 µm range with buffer-layer loaded piano-convex waveguide (BL-PCW) structures

A modified plano-convex waveguide structure is studied analytically and successfully applied to InGaAsP/InP lasers in the1.5- 1.6 mum wavelength region to realize stable transverse mode operation. The structure of these lasers is characterized by a standard buffer layer between an active layer and an upper cladding layer and a waveguide layer of varying thickness between the active layer and the substrate. A theoretical analysis of this structure showed that, for a given channel depth, increases in buffer-layer thickness give rise to larger maximum channel widths of the substrate for fundamental transverse mode operation. It was also shown that the optical confinement factor in the active layer decreases little as the buffer-layer thickness is increased from 0.1 to 0.3 μm. Buffer-layer loaded plano-convex waveguide lasers in the 1.5 μm range were prepared by liquid phase epitaxy and fundamental transverse mode operation up untilI = 2I_{th}was obtained. The dc threshold current was 100-300 mA and the differential quantum efficiency per facet was 10-15 percent. Continuous CW operation for over 2000 h at 25°C has been achieved.