T.P. Lee

Short-cavity InGaAsP injection lasers: Dependence of mode spectra and single-longitudinal-mode power on cavity length

Simple expressions are given to describe the lower and upper limits of the single-mode (single-frequency) power as a function of the cavity length for InGaAsP injection lasers. It has been found that the lower limit of the single-mode power is proportional to the cavity length, while the upper limit is inversely proportional to the cavity length. Thus, a short-cavity laser provides a favorable geometry for obtaining single mode output over a wide range of power levels and currents above threshold. The mode stability versus temperature is also improved by a short-cavity design. The theoretical results agree with our recent experiments on very-short-cavity ( 50-75 \mu m) stripe-geometry InGaAsP lasers, which have shown consistent single-mode output over wide current ranges.

Repetitively Q -switched light pulses from GaAs injection lasers with tandem double-section stripe geometry

Self-induced sustained pulsations have been observed in the light output from GaAs injection lasers with tandem double-section stripe geometry. The pulsing behavior of the laser is analyzed using a repetitively Q -switched model similar to that of Basov. Exact numerical solutions to the nonlinear rate equations have been obtained, which are to be compared with our experimental results and with the results in the earlier work of Basov et al. An approximate approach using a phase-plane analysis is described, which yields a simple way for the determination of the regions of excitation currents that leads to the sustained pulsing solution.

Picosecond frequency chirping and dynamic line broadening in InGaAsP injection lasers under fast excitation

We report the first observation of picosecond time‐resolved frequency chirping within each longitudinal mode of InGaAsP injection lasers under fast excitation. This transient phenomenon is responsible for the characteristic dynamic line broadening observed in the spectra of injection lasers under modulation. The frequency chirping invariably shows a ‘‘red shift’’ during the optical pulse, and is associated with the refractive index increase due to the carrier density decrease in the process of optical pulse generation. The dynamic line broadening will limit the single‐mode fiber transmission bandwidth even when single‐longitudinal‐mode injection lasers are used.

A stripe-geometry double-heterostructure amplified-spontaneous-emission (superluminescent) diode

A superluminescent diode (SLD) which simultaneously generates incoherent optical radiation and amplifies this radiation has been made from double-heterostructure (DH) Al x Ga 1-x As-GaAs material previously used to make lasers and light-emitting diodes having superior characteristics. The device configuration was similar to that of the stripe-geometry DH injection laser, except that optical feedback was suppressed by providing absorption for the backward waves in the cavity. A single-pass gain device resulted. The stripe contacts were 12.5 μm or 25 μm in width, and from 0.125 to 1.5 mm in length. The single-pass gain was found to be proportional to ( e^{\betaL} -1)/\beta , where β is a gain coefficient depending on the pump current and L is the stripe length. The structure was found to favor low-order modes, and the half-power radiation beam angle in the junction plane was measured to be 6-10\deg . The radiation field was slightly TE polarized, and it exhibited a spectral width of 50-85 A. When operated in the pulsed mode, the longest diodes (1.5 mm) radiated 60-mW peak power at a peak-current density of 6.5 kA/cm2(2.5-A current) and about 10 mW at 3 kA/cm2. The external differential quantum efficiency was 3.5 percent. Due to the relatively narrow radiation pattern of the SLD, more than 80 percent of the radiation from this source has been coupled into a multimode optical fiber with a numerical aperture of 0.63. These measured characteristics were found to conform to those calculated on the basis of a simple model.

Improved two-wavelength demultiplexing InGaAsP photodetector

The structure of the previously reported InGaAsP two-wavelength demultiplexing photodetector has been inverted to circumvent fabrication problems associated with dissolution of the lower-bandgap quaternary layer during the LPE growth of subsequent higher bandgap layers. In addition, lower doping levels have been achieved in the LPE layers. The result of these modifications has been devices with greatly improved optical and electrical characteristics. The long-wavelength cutoff has been extended to 1.6 μm.

Amplitude fluctuations and photon statistics of InGaAsP injection lasers

We experimentally show that the statistical distribution of the light output from an injection laser can be adequately described by the superposition of a coherent electric field and narrow-band Gaussian noise. A technique for measuring these fluctuations in the time domain with a resolution of 120 ps is described. The results for several InGaAsP lasers show that intensity fluctuations in an injection laser are rather large, so that the probability of finding the output below 50 percent of the average power in a 120 ps time interval can be as high as 10-5.

Dark current and breakdown characteristics of dislocation‐free InP photodiodes

The reverse I‐V characteristics of InP p‐n junction mesa photodiodes carefully fabricated to contain only a small number of dislocation etch pits were studied and compared. The results show that I varies with V approximately as expected for defect‐free junctions only when the mesas contain no visible etch pits.

Dual‐wavelength demultiplexing InGaAsP photodiode

We report the successful operation of a new photodiode structure capable of detecting and demultiplexing two wavelength bands simultaneously. Wavelength discrimination is achieved with a multilayer structure that incorporates p‐n junctions in two InxGa1−xAsyP1−y layers having different band gaps. In initial devices ’’crosstalk’’ between the photoresponses is less than −10 dB at each response peak.

Subnanosecond light pulses from GaAs injection lasers

Self-induced regular pulsations of the light intensity of GaAs injection lasers have been observed in pulse-driven diodes at liquid-nitrogen temperatures. The pulses occur at a repetition frequency of 500 MHz to 1 GHz, and the pulsewidth is less than 400 ps. These results are examined in the light of two theories that have been proposed to explain this kind of behavior in the GaAs laser. The basic differences between these two models are briefly described.

High avalanche gain in small‐area InP photodiodes

Extremely high avalanche gains, up to 2×104, accompanied by low leakage current and moderate excess avalanche noise, have been observed in small areas of diffused p‐n junctions in undoped n‐type InP. The results imply that dislocation densities much less than 104 cm−2 probably will be a prerequisite for practical long‐wavelength avalanche photodiodes in the InGaAsP/InP system.