Measurement of the fundamental modulation response of a semiconductor laser to millimeter wave frequencies by active-layer photomixing

The room-temperature modulation response of a GaAs/GaAlAs semiconductor laser (relaxation resonance frequency, vR=6.5 GHz) is measured to 37 GHz using the active-layer photomixing technique. The measured response function agrees with the theoretical ideal, and there is no indication of device parasitic effects. An ultrahigh-finesse Fabry–Perot interferometer is used to detect the optical modulation, which appears as sidebands in the laser field spectrum. With a moderately faster laser diode (i.e., vR~10 GHz), the modulation response should be measurable beyond 100 GHz

Low-temperature measurement of the fundamental frequency response of a semiconductor laser by active-layer photomixing

We use the active-layer photomixing technique to directly modulate the output of a GaAs semiconductor laser operating at temperatures as low as 4.2 K. The technique produces modulation with nearly perfect immunity to device parasitic effects, revealing the laser diode's intrinsic modulation response. At 4.2 K the parasitic corner frequency is estimated to be 410 MHz, yet the response appears ideal out to 15 GHz. We measure the dynamical parameters governing the response function, the relaxation resonance frequency, and the damping rate, and discuss their low-temperature behavior

Amplitude-phase decorrelation: a method for reducing intensity noise in semiconductor lasers

It has been shown that the method of amplitude-phase decorrelation can reduce the fundamental intensity noise floor of semiconductor laser light over a wide bandwidth by the ratio 1/(1+α^2), where α is the linewidth enhancement factor. The method uses a dispersive element to convert phase noise into intensity noise. This technique was recently demonstrated by reducing intensity noise from a DFB (distributed feedback) laser as much as 7 dB below its intrinsic level. In the present work, the authors extend these results by characterizing the frequency dependence of the noise reduction. Optimum reduction is achieved in the flat region of the spectrum and diminishes at higher frequencies approaching the relaxation resonance. The correlation properties of the fluctuations are also investigated

Intensity noise reduction in semiconductor lasers by amplitude-phase decorrelation

Detuned operation of a laser results in coupling of field amplitude and phase fluctuations. In a semiconductor laser, this coupling is known to be very large. We demonstrate that it can be used to significantly reduce intensity noise below its intrinsic limit

Parasitic-free modulation of semiconductor lasers

Active-layer photomixing is a technique for modulating semiconductor lasers with nearly perfect immunity to device parasitics. Measurements of the intrinsic modulation response of a laser diode using this technique at temperatures as low as 4.2 K are discussed. From these measurements, the temperature dependence of important dynamical parameters is determined. In addition, this provides a stringent test of the active-layer photomixing technique since parasitic response is degraded, while the intrinsic response is improved for low-temperature operation. At 4.2 K, the ideal intrinsic response is measured for frequencies as high as 15 GHz despite an estimated parasitic corner frequency of 410 MHz

Equivalent circuit model for active-layer photomixing: Parasitic-free modulation of semiconductor lasers

Direct modulation of a laser diode by active-layer photomixing is studied in terms of an equivalent circuit model. The model shows that this modulation technique achieves nearly perfect immunity to package, chip, and junction-related parasitic effects so that the measured modulation response reflects the intrinsic carrier-photon dynamics. The nonlinear gain effect is included in the model

Parasitic-free measurement of the fundamental frequency response of a semiconductor laser by active-layer photomixing

We report the measurement of the fundamental (intrinsic) frequency response of a GaAs semiconductor laser to 12 GHz by directly photomixing two optical sources in the active region of the laser. This novel technique reveals the underlying fundamental frequency response of the device as parasitic effects are avoided. Well beyond the relaxation resonance, the theoretically predicted 40 dB/dec signal rolloff is observed. Other features of the measured response function are also observed to be the theoretical ideal

Magnetic braking in young late-type stars: the effect of polar spots

The concentration of magnetic flux near the poles of rapidly rotating cool stars has been recently proposed as an alternative mechanism to dynamo saturation in order to explain the saturation of angular momentum loss. In this work we study the effect of magnetic surface flux distribution on the coronal field topology and angular momentum loss rate. We investigate if magnetic flux concentration towards the pole is a reasonable alternative to dynamo saturation. We construct a 1D wind model and also apply a 2-D self-similar analytical model, to evaluate how the surface field distribution affects the angular momentum loss of the rotating star. From the 1D model we find that, in a magnetically dominated low corona, the concentrated polar surface field rapidly expands to regions of low magnetic pressure resulting in a coronal field with small latitudinal variation. We also find that the angular momentum loss rate due to a uniform field or a concentrated field with equal total magnetic flux is very similar. From the 2D wind model we show that there are several relevant factors to take into account when studying the angular momentum loss from a star. In particular, we show that the inclusion of force balance across the field in a wind model is fundamental if realistic conclusions are to be drawn from the effect of non-uniform surface field distribution on magnetic braking. This model predicts that a magnetic field concentrated at high latitudes leads to larger Alfven radii and larger braking rates than a smoother field distribution. From the results obtained, we argue that the magnetic surface field distribution towards the pole does not directly limit the braking efficiency of the wind.Comment: 11 pages, 10 figures, accepted in A&

The optical gain lever: A novel gain mechanism in the direct modulation of quantum well semiconductor lasers

A new gain mechanism active in certain quantum well laser diode structures is demonstrated and explained theoretically. It enhances the modulation amplitude produced by either optical or electrical modulation of quantum well structures. In the devices tested, power gains of 6 dB were measured from low frequency to frequencies of several gigahertz. Higher gains may be possible in optimized structures