The integration of photonic clocking in microprocessors is anticipated to occur during the 2008-2012 high-volume manufacturing (HVM) cycle. Though photonic clocking can be achieved through electronic modulation or actively mode-locking a laser, a more cost-effective and better solution would be to use internal cavity passively mode-locked semiconductor lasers. Not only do these lasers offer low-cost, simplicity, and ease of integration, but prototypes that are amenable to HVM are currently available. We present such a laser that is scalable by design to clock rates of 9 to hundreds of GHz and wavelengths in the 800 to 1100+ nm range. These lasers utilize internal saturable absorber(s) to passively mode-lock a semiconductor laser with relatively high peak powers. Experimental results from these lasers show an RF spectrum signal peak that is at least 40 dB above the noise floor with a -10 dB width of <1 MHz. The RMS jitter as determined by an oscilloscope with a precision timebase module was found to be ~1 ps which is among the best for this type of laser. Autocorrelation was used to confirm mode-locking and pulse width. In addition to experimental data, a theory and discussion on how the different characteristics of these lasers can be tailored for various commercial applications such as microprocessor clocking will be presented.
We perform maximum range calculations for surface sensing direct-detection mid-infrared Random-Modulation Continuous- Wave (RM-CW) lidar, and propose a system's figure of merit. In this type of lidar, noise can be additive and determined by detector's noise given by its specific detectivity D, area, and integration time. Fundamental limits of the sensing range are then imposed by diffraction, provided that the Background-Limited Infrared Photodetection (BLIP) limit (controlled by the field-of-view) is not exceeded. There is no dependence on a specific pseudorandom code (PRC) used since noise spectral density is assumed to be independent of frequency. Under most favorable conditions, including no losses due to optics or atmospheric attenuation, an object of angularly uniform unity reflectance and size not less than the laser beam can be detected within 1s with a 20-cm diameter telescope and a signal-to-noise ratio of one from a distance of about 200 km if a cooled detector and a laser emitting 500 mW cw at 5 micrometers are used. For an integration time of 0.1 ms (which is well below a typical atmospheric correlation time), or a room-temperature detector, the maximum range reduces to about 20 km. As the technology of Quantum-Cascade lasers (QCLs) advances, the maximum range of several tens of kilometers can be expected in this type of lidar with all-thermoelectrically-cooled semiconductor components, and, perhaps after some modifications in the technique, about 1 km with all-room-temperature components.
We propose and analyze a new modulation sequence for Random- Modulation Continuous-Wave (RM-CW) lidar. It is compared to known sequences, and shown to have significantly better, and nearly-ideal, signal properties. Namely, the cross- correlation function of this new sequence - named the AA1- sequence - consists of single peaks separated by zeros. Consequently, unlike the A1- and A2-sequence, it is immune to interference caused by backscattering from different ranges. Also, since the demodulation sequence is balanced, the new sequence, unlike the M-sequence, does not require the low laser output power to be zero to maintain desired cross-correlation properties; in lidar using a semiconductor laser as a transmitter, this would eliminate coupling between the demodulated signal amplitude and the emission wavelength, and thus facilitate wavelength-dependent measurements. Furthermore, we have calculated the post- demodulation signal-to-noise ratio in the presence of an additive noise of arbitrary power spectra density - it is applicable in all cases where the noise does not depend on the signal, which is typical in direct-detection mid-IR lidar. The results show that in baseband transmission all these sequences have similar noise properties, except that the M-sequence - due to its imbalance property - has a much stronger near-zero-frequency noise pickup, which results in significantly worse noise performance in practical systems. Therefore, we claim that the new modulation sequence would yield superior performance in RM-CW lidar.
Continuous-wave (CW) room temperature operation of InGaN/GaN multi-quantum well (MQW) lasers is reported. Far-field beam divergence as narrow as 13 degrees and 20 degrees for parallel and perpendicular directions to epilayer planes were measured, respectively. The MQW lasers showed strong beam polarization anisotropy as consistent with QW laser gain theory. Dependencies of threshold current on cavity-length and temperature are also consistent with conventional laser theory. No significant degradation in laser characteristics was observed during lifetime testing for over 140 hours of CW room temperature operation.
We report metalorganic chemical vapor deposition-grown double heterostructure and multiple quantum well InAsSb/InAsSbP laser diodes emitting at 3 to 4 micrometers and light emitting diodes up to 5 micrometers . Maximum output power up to 1 W was obtained from a MQW laser with stripe width of 100 micrometers and cavity length of 700 micrometers for emitting wavelength of 3.6 micrometers at 90 K. Maximum operating temperature up to 220 K with threshold current density of 40 A/cm2 at 78K were achieved from the double- heterostructure lasers emitting at 3.2 micrometers . The far-field beam divergence as narrow as 24 degrees was achieved with the sue of higher energy gap barrier layers, i.e., lower effective refractive index, in MQW active region. We also discuss the effect of composition-fluctuation in the InAsSb active region on the gain and threshold current of the lasers.
One of the main disadvantages of free-running semiconductor lasers in spectroscopy is gaps in their tuning ranges. Due to these gaps only part (about 30%) of the spectrum can be covered in a typical temperature tuning. A method of avoiding such gaps in a multiple quantum well type STC LT50A-03U laser is reported in this contribution. One can expect that the results may be extended to the whole class of MQW lasers. In other types of diode lasers spectrum coverage of about 45% (instead of typical 30%) can be obtained in the proposed way. Furthermore, a qualitative relationship between wavelengths available this way and parameters of a laser junction can be derived within a simple model of mode-hopping behavior.
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