Luis Figueroa

High-frequency characteristics of GaAlAs injection lasers

This paper discusses the results of an extensive experimental study of the high-frequency characteristics of several commercial (GaAl)As injection lasers. The lasers tested included the Hitachi buried heterostructure (BH), the Hitachi channel substrate planar (CSP), the Mitsubishi transverse junction stripe (TJS), the General Optronics stripe (GO), and the RCA constricted double heterostructure (CDH). We find that the maximum practical analog modulation frequency f o is in the range of 5 GHz. This practical modulation frequency is limited by a combination of laser-relaxation resonance, laser parasitics, and drive current necessary for long-life operation. The relationship between laser-relaxation oscillation resonance frequency f o and drive current can be expressed as f_{o} = K(I/Ith - 1)^{1/2} GHz where I and Ith are the laser drive and threshold currents, respectively. The value of K is experimentally determined to lie in the range 3.0 K j and a capacitance C j . The product of R_{j}C_{j} has been experimentally determined for both the Hitachi CSP and BH laser and lies in the range of 1-1.1 \times 10^{-10} s. The effect of laser mount and packaging can also be included in the equivalent circuit by the addition of series inductors and shunt capacitors. Lastly, all injection lasers studied displayed a dip in the small signal modulation response prior to the onset of the relaxation oscillation resonance. In particular, there is a pronounced dip even for lasers with good lateral carrier confinement at dc, such as the buried heretostructure (BH). Previous workers have attributed the dip in the modulation response as resulting from either lateral diffusion of injected carriers, or the effect of junction capacitance. Our experimental results indicate that while the effect of the junction capacitance may play some role in explaining the dip for the BH laser, it is not the dominant mechanism. We speculate that lasers with good lateral carrier confinement at dc (BH) may suffer a significant degradation of carrier confinement at gigahertz rates. A simple qualitative model is proposed to explain this mechanism and methods for reducing the effect are discussed.

Control of mode behavior in semiconductor lasers

In this paper, aspects important to lateral-mode stability and single longitudinal-mode operation in the index-guided semiconductor lasers are examined and analyzed. These include the modal properties of the guiding structure and the current distribution in the active region. Waveguiding mechanisms in various structures are analyzed and approximate expressions for the number of lateral modes are obtained. Possible modes in rectangular dielectric waveguides are presented and discussed. The problems of nonsatisfaction of the boundary condition and ambiguity in the field polarization are pointed out, which could be important in structures with equally strong guiding and approximately equal dimensions in the transverse and lateral directions. New equations governing the current distribution in the active region are presented. Our work differs from the previous work in that the voltage path-independence condition, in addition to the current conservation condition, is taken into account in the equations. The discussions outlined above prepare the background for a discussion of the factors affecting lateral-mode stability and single longitudinal-mode operation. It is shown that a positive gain difference \Delta g must be maintained in the lateral direction for lateral-mode stability, even though a positive index difference \Delta n exists. The effect of nonuniform gain on longitudinal modes is examined. It is shown that a ten percent variation in carrier concentration can result in a shift in peak-gain wavelength about 25 A. For single longitudinal-mode operation, it is important to keep \Delta g within a certain limit, and the effect of spatial gain variation on spectral gain profile must be considered. The above ideas for maintaining lateral-mode stability and single longitudinal-mode operation are implemented in the narrow-channel DCC-CSP laser. Experimental results are reviewed in the light of the theoretical considerations. For optimal performance over an extended laser-power range, considerations must be given in the design of an index-guided laser to the combined effects of current spreading, spatial hole burning, and carrier diffusion, so as to maintain a positive but small \Delta g over the desired operation range.

Study of mode locking in (GaAl) as injection lasers

This paper describes the important experimental parameters affecting the mode locking of a variety of (GaAl)As injection lasers operating in an external optical cavity. We find that short detector limited pulses (less than 60 ps) and 100 percent modulation depth can only be obtained using lasers which exhibit either an anomalous narrow-band noise or self-pulsations. Little or no mode locking is observed in lasers having the normal broad noise resonance. The observed amplitude, pulsewidth, and frequency of the mode-locked pulses are correlated to the degree of self-pulsation and the external cavity length. The experimental results obtained are in qualitative agreement with a model which uses the rate equations modified by either electron traps or saturable absorbers and a delayed feedback term. Our results appear to imply that mode locking in injection lasers is related to laser defects and is very similar to passive mode locking in dye lasers.

Intensity self‐pulsations in (GaAl)As injection lasers operating in an external cavity

We present an experimental and theoretical study of a self‐pulsing (GaAl)As injection laser operating in an external cavity. We have observed suppression of the self‐pulsations when the external cavity is in the range 6<L<10 cm. Suppression of self‐pulsations can also be obtained by using a multimode graded index optical fiber as the external resonator. These results can be explained by a model which includes the effects of an external cavity and electron trapping. For long cavity lengths, the self‐pulsation frequency locks to an external cavity harmonic with no significant quenching, as observed in some earlier experiments.

Generation and quenching of intensity pulsations in semiconductor lasers coupled to external cavities

The behavior of self-pulsing and nonpulsing lasers coupled to external cavities is investigated experimentally and theoretically. We investigate the dependence of the pulsation characteristics on the external cavity length using a saturable absorber model for self-pulsing lasers. It was found that quenching of self-pulsation occurs only for a certain limited range of external cavity length, and the frequencies of external-cavity induced pulsations lies within a certain range determined by the coupling coefficient. Small-signal analysis allows these ranges to be derived analytically. Hitherto, complex pulsation phenomena can be explained very intuitively by interpreting the combined laser-external cavity system as a microwave oscillator with a limited gain band and discrete mode structure.

Studies of (GaAl)As injection lasers operating with an optical fiber resonator

The characteristics of an optical fiber external resonator in conjunction with (GaAl)As stripe geometry lasers are described. We have observed a 6–10% reduction in the threshold current and have obtained 150 ps pulses at gigahertz repetition rates. The fiber resonator has also been used to quench self‐pulsations in a (GaAl)As injection laser. In order to explain many of our results we have used a model that uses the conventional semiconductor rate equations modified by the addition of saturable electron traps and the effects of the external cavity. Our results predict many of the self‐locking effects observed in injection lasers operating in an external cavity. Furthermore, the degree of self‐locking will be a strong function of the external cavity length and the density of saturable absorbers.

Predicting diode laser performance

Predictions of the threshold current density of GaAs/AlGaAs graded refractive index (GRIN), separate confinement heterostructure (SCH), single potential well (SW) diode lasers at 25 degree(s)C and 125 degree(s)C using strict k-selection theory are made. A reasonable fit to the experimental data at both temperatures can be obtained without including carrier scattering. It is concluded that good predictions of threshold current density and differential quantum efficiency can be made provided one knows how to predict the temperature dependence of internal quantum efficiency.

Buried-ridge striped planar GaAlAs/GaAs lasers with a wide range of effective index steps

A novel self‐aligned buried‐ridge striped planar (BRSP) GaAlAs/GaAs diode laser structure is presented. The structure was grown by two‐step liquid phase epitaxy using preferential growth and melt etch. A Ga1−y Aly As layer with a wide range of Al concentration (y>0.05) is selectively grown outside the ridge and can be an effective melt‐etching mask. As a result, the BRSP structure can incorporate virtually all effective index steps, i.e., the positive, negative, and complex index steps. The threshold current of the BRSP structure is between 70 and 100 mA. The BRSP structure may prove to be of importance in the realization of linear array lasers.

Mode Locking Of (GaAI)As Injection Lasers

The important experimental parameters affecting the mode locking of a variety of (GaAl)As injection lasers operating in an external optical cavity are described. We find that short detector-limited pulses (less than 60 psec) with 100% modulation depth can only be obtained using lasers that exhibit either an anomalous narrowband noise resonance or self pulsations. Little or no mode locking is observed in lasers having the normal broad noise resonance. The maximum frequency of the mode-locked pulses is ≈1 GHz and is limited by the laser and not the external cavity. The observed amplitude, pulse width, and frequency of the mode-locked pulses are correlated to the degree of self-pulsation and the external cavity length. The experimental results are in qualitative agreement with a model that uses the rate equations modified by either electron traps or saturable absorbers and a delayed feedback term. Our results appear to imply that mode locking in (GaAl)As injection lasers is related to saturable absorbing centers and is very similar to passive mode locking in dye lasers.