Μm InGaAsP Laser Research Articles

Use of external optical feedback to stabilize multimode hopping and reduce intensity noise in long-wavelength semiconductor laser

We report on the possibility of applying optical feedback (OFB) from a short external cavity to change the multimode hopping oscillation in long-wavelength semiconductor laser to single-mode oscillation. We characterize the influence of OFB on the modal oscillations and the associated noise spectrum of the multimode laser. The study is based on a modified time-delay multimode rate-equation model of the laser that includes mechanisms of spectral gain suppression along with OFB induced due to multiple reflections by an external reflector. The study is applied to 1.55 μm-InGaAsP laser that exhibits multimode hopping in its solitary version and supports wide bandwidth. The noise is evaluated in terms of the relative intensity noise. We show that strong OFB changes impact of the asymmetric gain suppression on the mode structure of the laser so that it changes the side mode suppression ratio (SMSR) and induces jumping of the strongest modes to longer or shorter wavelengths in random-like fashion. The change of SMSR of the laser occurs mainly when strong OFB induces multiperiodic and chaos oscillations. We specify the ranges of OFB over which SMO is induced. The low-frequency noise is lowest when the laser maintains CW operation or exhibits period-1 oscillations, while it is maximized when the laser dynamics are chaotic.

Coupling of large optical loss with Auger recombination in 1.3 μm InGaAsP lasers investigated using hydrostatic pressure

AbstractInGaAsP lasers with different losses have been investigated under hydrostatic pressure at room temperature. It is shown that, for large loss devices, the threshold current decreases more rapidly with increasing pressure than the expected variation of the typical direct band‐to‐band Auger recombination currents. We also measured the spontaneous emission in a large loss device to determine the pressure dependence of the respective recombination current component at laser threshold. It is found that the Auger current reduces strongly and the radiative current also decreases with increasing pressure. This is attributed to a reduction of the total loss coupled with Auger recombination with pressure.

Composition Control of InGaAsP Laser Structures Using In-Situ Emissivity Corrected Pyrometry During Epitaxial Growth

Wafer temperature during metal-organic chemical vapor deposition growth strongly influences the composition, and therefore, photoluminescence wavelength of InGaAsP layers when using AsH3 and PH3 precursors. Emissivity corrected pyrometry was used to measure and control the wafer temperature during epitaxial growth of 1.5 µm InGaAsP laser structures. Control of laser structure wavelength was demonstrated using an Emcore D180 LDM reactor. Average peak wavelength for five, six wafer runs had a standard deviation of 2.1 nm indicating excellent repeatability as long as wafer temperature was identical from run to run. Composition control was further demonstrated by intentionally reducing the wafer temperature during two successive growth runs. Average peak PL wavelength increased from 1522 nm to 1530 nm by reducing RealTemp wafer temperature 2°C from 607.7°C to 605.7°C during active region growth, corresponding to a 4 nm shift of wavelength for a 1°C change.

InGaAsN/GaAs heterostructures for long-wavelengthlight-emitting devices

We report on the growth and properties of InGaAsN/GaAsheterostructures and on their applications for lasersemitting at λ≈1.3 µm. Material growth wasperformed by molecular beam epitaxy using an RF plasma source.Broad area and ridge waveguide (RWG) laser structures based on suchquantum wells (QWs) exhibit performances that can compete with those of1.3 µm InGaAsP lasers. In particular, we have achieved300 K operation of broad area lasers at 1.3 µm withthreshold current densities down to 500 and 650 A cm-2 for 800 µm long, single and triple QWstructures. Similar structures with heat-sinking at10 °C yield a maximum CW output power of up to8 W.RWG lasers have thresholds down to 11 mA and show CW operationup to 100 °C with a T0 of up to 110 K.

A comparison of algainas and InGaAsP-based 1.3 μm semiconductor lasers using high pressure

We have compared the effect of hydrostatic pressure on the threshold current, Ith, and lasing energy, Elase, of 1.3 pm quantum-well devices based upon AlGaInAs and InGaAsP. Whilst we observe a very similar dependence of Elase on pressure for the two materials, we measure strikingly different variations of Ith. By applying pressure to 1.3 μm InGaAsP lasers, Ith typically decreases by ∼ 10% over 1 GPa consistent with the reduction of Auger recombination, which forms ∼ 50% of Ith at room temperature. However, for the 1.3 μm AlGaInAs-based lasers, we observe an increase in Ith by ∼ 8% over the same pressure range. From these results we conclude that non-radiative recombination accounts for only ∼ 20% of Ith in AlGaInAs-based devices. This is in good agreement with previous temperature dependence measurements and shows why AlGaInAs-based devices exhibit a reduced temperature sensitivity of Ith which is very important for telecommunications applications.

Experimental observation of mode behavior in erbium-doped optical fiber ring laser

Two-polarization mode oscillation is observed in an erbium-doped optical fiber ring laser. This phenomenon was verified by the existence of the polarization hole burning effect in this optical fiber ring laser. The laser is a ring laser operating in the region of 1.55 μm, pumped by a 1.48 μm InGaAsP laser diode. Mode-hopping and mode-competition instabilities due to spatial hole burning and inhomogeneous spectral broadening were eliminated by using an ultra-narrow-bandwidth optical fiber Bragg grating and a polarization independent optical isolator. The oscillation in the two-polarization mode operation is remarkably stable at higher pump powers. The application of two-polarization mode oscillation for temperature sensing is described. The sensing is novel in that the change of double refraction with temperature and the frequency detection are combined. In this method, the detection sensitivity with temperature rise was about 100 Hz/°C at around 40°C.

Nonmonotonic Behavior of the Threshold Current of Compressively Strained 1.3 μm InGaAsP Lasers with Hydrostatic Pressure

AbstractHydrostatic pressure measurements of the threshold current of 1.3 μm compressively strained InGaAsP multiple quantum well lasers show an initial decrease up to 1.2 GPa followed by an increase at higher pressures. This behavior is explained by considering the changes in the differential gain and optical losses determined from measurements of the spontaneous emission and of the external differential efficiency as a function of pressure, respectively.

8-band k.p theory of the material gain of strained tetrahedral semiconductors. Application to 1.3 µm-InGaAsP lasers subject to additional external uniaxial stress

Numerical and experimental results are presented for bulk 1.3 /spl mu/m-InGaAsP on InP subject to internal biaxial strain in the xy-plane (active layer, strain axis in growth or z direction) and/or external stress along the lateral, x direction. This uniaxial stress by an external force acting perpendicular to the built-in biaxial strain provides an additional degree of freedom for studying the strain dependence of electron states and, thus, of device properties. The material gain is calculated using a novel method for efficient Brillouin-zone integration. The maximum linear gain is parametrised as g(n)=a(n-n/sub l/) for TE- and for TM-polarised light, and the strain dependence of differential gain a and of transparency density n is calculated and discussed in terms of symmetry. The theoretical results compare well with measurements of threshold currents and emission-wavelength differences at TE-TM switching of ridge-waveguide laser diodes.

Steady state model for facet heating leading to thermal runaway in semiconductor lasers

A steady state model is presented which provides new insight into the thermal runaway process that leads to catastrophic damage of semiconductor lasers. We show that thermal runaway is preceded by a situation where two self consistent thermal steady state solutions exist at low output power, one stable and one unstable. When the output power is increased, the two solutions degenerate and disappear which means that the laser will enter thermal runaway. The steady state model consists of two parts: a three dimensional thermal model and a one dimensional model for the carrier diffusion towards the facet. The temperature dependence of both the heat sources and the thermal conductivity play the crucial role. Also ordinary bulk heating is shown to be an important factor. Both 0.88 μm GaAs lasers and 1.5 μm InGaAsP lasers are discussed and minimum values of surface recombination and output power needed for thermal runaway are given. Thermal runaway in GaAs lasers can be explained by the model for realistic values of surface recombination. However, the calculated values of needed output power are significantly higher than what is experienced in reality. Possible explanations for this discrepancy are given.

Femtosecond Erbium-Doped Fiber Lasers and a Soliton Compression Technique

The lasing characteristics of an erbium-doped fiber laser with a nonlinear amplifying loop mirror (NALM) were investigated in detail. The NALM was pumped by 1.48 µm InGaAsP laser diodes and an output pulse as short as 124 fs was obtained at 1.56 µm. The 124-fs pulse was shortened to 50 fs by using adiabatic soliton narrowing and high order soliton compression. Soliton narrowing was achieved by amplifying the soliton pulse adiabatically with an erbium-doped fiber amplifier, and a short length of dispersion-flattened fiber was used for soliton compression. Soliton narrowing enabled us to succeed in generating a 98-fs pulse directly from a modified erbium-doped fiber laser.

Determination of the gain nonlinearity time constant in 1.3 μm semiconductor lasers

By comparison of the measured K factors (ratio of the damping factor to the square of the resonance frequency) of distributed feedback and Fabry–Perot lasers, it is found that the relaxation time associated with nonlinear gain for 1.3 μm InGaAsP lasers is about 0.1 ps. This short time constant is consistent with spectral hole burning being the dominant process responsible for the nonlinear gain.

Extremely high-frequency (24 GHz) InGaAsP diode lasers with excellent modulation efficiency

1.3 μm InGaAsP lasers with 3 dB bandwidths of 24 GHz and intrinsic resonance frequencies in excess of 22 GHz have been successfully fabricated. This is the highest bandwidth ever reported for a semiconductor laser, and the highest resonant frequency for InGaAsP lasers. Excellent modulation efficiencies are observed to high frequencies.

Efficient Er3+-doped optical fiber amplifier pumped by a 1.48 μm InGaAsP laser diode

Optical gain characteristics of an Er3+ -doped silica fiber have been studied by end pumping with a 1.48 μm InGaAsP high-power laser diode. A gain as high as 12.5 dB was obtained for an absorbed pump power of 16 mW with a 3-m-long fiber. By constructing an Er3+ fiber ring cavity with a 3 dB single-mode fiber coupler, we have obtained continuous wave laser oscillation at a wavelength of 1.553 μm.

Addendum: Measurement of dg/dN and dn/dN and their dependence on photon energy in Λ = 1.5 μm InGaAsP laser diodes

Addendum: Measurement of dg/dN and dn/dN and their dependence on photon energy in Λ = 1.5 μm InGaAsP laser diodes

Ultra-high-speed modulation of 1.3-µm InGaAsP diode lasers

The differential gain of 1.3 μm InGaAsP lasers is found to be a strong function of the active layer doping level. Using devices with doping enhanced differential gain and short cavity lengths, a modulation bandwidth of 15 GHz is achieved. The parasitic shunt capacitance which often severely limits the modulation bandwidth of diode lasers is greatly reduced by using a 0.4 μm thick oxide isolation layer and a device structure which minimizes parasitic capacitances. It is found that nonlinear gain saturation predominantly determines the damping factor of these devices, and thus greatly influences their modulation characteristics. The contribution of nonlinear gain to the FM modulation index and relaxation oscillation damping factor is calculated and measured. The nonlinear gain contribution to the damping factor obtained from the measured FM modulation index is used to predict the optical intensity modulation response. Excellent agreement between prediction and observation is obtained. These data directly show that the damping factor of multilongitudinal-mode lasers is smaller than that of single-mode lasers, suggesting that wider modulation bandwidths can be achieved with multimode devices than with single-mode devices.

High-speed InGaAsP constricted-mesa lasers

Recently, the bandwidths of semiconductor lasers, detectors, and optical transmission systems have been dramatically increased. The considerations used to achieve a record 26.5 GHz bandwidth in a 1.3 μm InGaAsP laser at -60°C are described here. The small-signal modulation characteristics of a number of different laser structures are compared. Several large-signal modulation experiments are described with emphasis on the laser response to 8 Gbit/s modulation.

Picosecond pulse analysis of gain-switched 1.55 µm InGaAsP laser

Results of computer simulation of gain-switched pulses from InGaAsP lasers emitting at 1.55 μm are reported. Multimode rate equations are solved for short triangular current pulses of large amplitude superimposed on a dc bias close to threshold. Lasers of conventional length (200μm) and with short cavity (50 μm) are analyzed. The effect of wavelength chirp on time-resolved and the time-averaged multimode spectra is demonstrated. Optical pulses as short as 10 ps are predicted for short-cavity lasers with reflective coatings. While the number of longitudinal modes in gain-switched spectrum is efficiently reduced for short-cavity lasers, the dynamic linewidth of each mode remains of a similar magnitude (up to 140 GHz) as for standard-cavity lasers. It is expected that gain saturation effects could reduce the calculated values of dynamic linewidth.

Linewidth reduction of a semiconductor laser by electrical feedback

An electrical feedback technique was proposed to stably reduce the linewidth of a semiconductor laser without changing its cavity structure. Calculations and experiments were carried out to reduce the linewidth of a 1.5 μm InGaAsP laser (DFB type) according to the following procedure. A compact Fabry-Perot interferometer was used as a freqeuncy discriminator. The minimum attainable linewidth, limited by the detector noise, was estimated as being narrower than 1 kHz when the reflectance of the interferometer used was higher than 0.9. The minimum linewidth obtained in the experiment was 330 kHz, which was 15 times as narrow as in the case of a free-running laser. The improvements of this experimental result can be expected by simultaneously reducing the AM noise of the laser.

Locking range and stability of injection locked 1.54 µm InGaAsp semiconductor lasers

Measurements of the intensity of an injection locked 1.54 μm InGaAsP laser are reported. The change in intensity of the locked laser across the locking range is quite asymmetric, with a shape that agrees well with the theory of Lang. A linewidth parameter of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\alpha = 6 \pm 1</tex> was determined from the magnitude of the locking range. The injection locked laser was found to be unstable on the high frequency end of the locking range. The physical origin of this instability is explained in terms of a laser intensity change altering the phase of the laser field relative to that of the injected field.

Analysis of ridge-waveguide distributed feedback lasers

An analysis concerning the coupling efficiency and the threshold current density is presented for a ridge-waveguide distributed feedback laser. Particular attention is paid to the grating efficiency that is different under and beyond the ridge region because of different lateral environments and generally different corrugation depths. The threshold current density and its optimization are discussed for 1.55 μm InGaAsP lasers after considering the design parameters, such as the ridge width and the layer thicknesses. Both the TE and TM modes are considered. A narrow ridge helps to reduce the device threshold if the grating under the ridge is absent or is partially washed out during the ridge-overgrowth procedure.