Analysis of symmetric Y-junction laser arrays with uniform near-field distribution

The analysis of a Y-junction laser array is presented. It is shown that the lowest-order cavity mode of this array has the lowest threshold gain and is thus preferentially excited. This mode has a uniform near-field distribution. Compared with the sinusoidal near-field distribution of the supermodes of the evanescently coupled laser arrays, this uniformity not only can delay the onset of the spatial-hole burning, which is the source of unstable far-field characteristic at high power level, it also allows for higher total power output before catastrophic mirror damage.     

Observation of sustained self-pulsation in CW operated flaredY-coupled laser arrays

The time evolution of the near-field radiation patterns of CW operated inphase locked flared `Y' coupled diode laser arrays has been observed with a streak camera. The arrays exhibit sustained self-pulsation at frequencies as high as 4 GHz     

Frequency control of a single-frequency fiber laser bycooperatively induced spatial-hole burning

A simple frequency control method of a single-frequency fiber laser is proposed and demonstrated. The mechanism of the frequency control arises from cooperatively induced spatial-hole burning (SHB) in a saturable absorber in the laser cavity. The SHB is formed by an external frequency-stabilized light source and the lasing light. Consequently, the lasing frequency can be tuned to the external light frequency. Moreover, narrow-linewidth operation is possible even when an external-source with a broad-linewidth is employed     

1.1-W continuous-wave 1480-nm semiconductor lasers with distributedelectrodes for mode shaping

Diffraction-limited high-power devices may suffer from self-focusing effects due to nonuniform gain saturation. In this letter, we propose the concept of the distributed electrode, which allows one to improve the modal behavior of these lasers and to reduce spatial-hole burning effects by preferentially localizing current injection in the center of the structure, therefore shaping the optical mode. Utilizing this concept, we have realized unstable cavity lasers exhibiting single-lobe far-field patterns. We report the first realization of flared unstable cavity lasers emitting at 1480 mm with maximum output powers up to 1.1-W continuous-wave and external efficiencies as high as 0.45 W/A     

High-frequency pulsations in DFB lasers with amplified feedback

We describe the basic ideas behind the concept of distributed feedback (DFB) lasers with short optical feedback for the generation of high-frequency self-pulsations and show the theoretical background describing realized devices. It is predicted by theory that the self-pulsation frequency increases with increasing feedback strength. To provide evidence for this, we propose a novel device design which employs an amplifier section in the integrated feedback cavity of a DFB laser. We present results from numerical simulations and experiments. It has been shown experimentally that a continuous tuning of the self-pulsation frequency from 12 to 45 GHz can be adjusted via the control of the feedback strength. The numerical simulations, which are in good accordance with experimental investigations, give an explanation for a self-stabilizing effect of the self-pulsations due to the additional carrier dynamic in the integrated feedback cavity.     

Self-pulsation in an InGaN laser-theory and experiment

Room-temperature operation of self-pulsating InGaN lasers was obtained at a wavelength of 395 nm. The laser structure consists of a multiquantum-well InGaN active layer and a p-type InGaN single-quantum-well saturable absorber. The frequency range of the self-pulsation was from 1.6 to 2.9 GHz. The experimental results were well explained with our theoretical analysis. We found that features of the saturable absorber strongly affect the self-pulsation. Influence of device and material parameters on the laser dynamics was also investigated.     

Polarization behavior of birefringent multitransverse mode vertical-cavity surface-emitting lasers

An analysis has been undertaken of the effect of birefringence on the selection of polarization states of a weakly index guided vertical-cavity surface-emitting laser (VCSEL) supporting both a fundamental and a first-order transverse mode. It is shown that for small index steps polarization switching due to spatial-hole burning effects can occur. For larger index steps it is found that higher order modes can emerge which are orthogonally polarized to the dominant polarization of the fundamental mode.     

近场热辐射的最新研究进展

主要从理论数值模拟和近场辐射实验研究的角度介绍了近几年在近场热辐射传热方面的最新研究成果。理论研究的焦点主要集中在石墨烯复合材料、人工加工或合成超材料等方面的传热研究。实验研究的焦点是实验室基于纳米尺度近场热辐射测量的设备制造与方法创新。目前实验上已经实现了最小距离仅为2 nm的极近场热辐射测量。近场热辐射的进一步研究可为热光伏、辐射制冷以及高效能源收集应用提供理论基础。     

Role of the conductivity of the confining layers in DH-laser spatial hole burning effects

Spatial hole burning (which has been advanced as a cause of kinking, multimode operation, self-pulsation, and other effects in double heterostructure lasers) is shown theoretically to be appreciably suppressed by highly conductive confining layers.     

Distinction between multimoded and singlemoded self-pulsations-inDFB lasers

DFB lasers with split contacts are shown, by large signal dynamic modelling, to self-pulsate at gigabit frequencies. Two different self-pulsation schemes are discussed: where the laser switches between the higher and lower stop band modes, and where the laser pulsates around a single mode. The second scheme can yield self-pulsation frequencies beyond 20 GHz. Comparisons are made with experimental results     

半导体激光器的自脉动与被动锁模

利用普适GaAlA3双异质结半导体激光器，观察到被动锁模现象。讨论了半导体激光二极管的自脉动、外腔中的诱导自脉动以及被动锁模间的区别与联系。     

Self-pulsation in two-section DFB semiconductor lasers and itssynchronization to an external signal

A model of self-pulsation in two-section distributed feedback (DFB) lasers without a saturable absorber is developed by using generalized rate equations. The introduction of an effective differential gain in our model allows us to take into consideration both material and structural effects. The self-pulsation conditions are derived from a linear stability analysis. A mechanism based on a negative effective differential gain is proposed to explain the origin of self-pulsation in such lasers. By considering an injected optical signal, the optical synchronization of self-pulsating lasers is studied using nonlinear simulations. This leads to the determination of some locking-range properties, which are then compared to experimental and analytical results     

Near-field gain of a horn and an open-ended waveguide: Comparison between theory and experiment

Generating a standard electromagnetic field requires knowledge of the gain of the transmitting antenna. The theory and supporting experimental measurements of the near-field gain of a pyramidal horn and an open-ended waveguide (OEG) at 450 MHz are given. The empirical near-field gain for the OEG is derived from experimental results obtained by a two-antenna method at about 2 GHz. The theoretical nearfield gain for the rectangular pyramidal horn is derived from Schelkunoff's formula. Two independent near-field gain measurements of these antennas are made using a three-antenna method and a transfer-standard-probe method. The discrepancy between theoretical and experimental results is typically less thanpm 1dB.     

Antenna pattern comparison between an outdoor cylindrical near-field test facility and an indoor spherical near-field antenna test facility

A new spherical near-field probe-positioning device has been designed and constructed, consisting of a large 5.0 meter fixed arc. This arc has been installed in a near-field test facility, located at Alenia Marconi Systems, on the Isle of Wight, UK. As part of the near-field qualification, testing was performed on a ground-based radar antenna. The resultant patterns were compared against measurements collected on the same antenna on a large outdoor cylindrical near-field test facility, also located on the Isle of Wight [F. Steiner et al., Jan. 1994]. These measurements included multiple-frequency measurements and multiple pattern comparisons. This paper summarizes the results obtained as part of the measurement program, and includes discussions on the error budgets for the two ranges, along with a discussion of the mutual error budget between the two ranges.     

Enhanced single-mode operation in multisection DFB lasers with bothlocalized and distributed phase shift

Multisection distributed feedback lasers with both localized (λ/8 phase shift) and distributed phase shift (produced by a phase-adjustment region) are investigated theoretically. It is shown that such a mixed phase shift, combined with a stronger coupling coefficient in the center of the cavity, may enlarge the threshold gain margin as well as the output optical power, enhance the intra cavity field flatness and produce an effective index profile leading to a low sensitivity to spatial-hole burning (SHB) (with respect to conventional λ/4 phase-shifted DFB lasers). As a consequence, the side-mode suppression ratio (SMSR) is improved and single-mode operation up to high bias currents is maintained     

Chaotic self-pulsation and cross-modulation in awavelength-selective external-cavity laser diode

Chaotic self-pulsation in a single wavelength external-cavity laser diode is observed. It is shown that the self-pulsation is caused by interdependencies between the optical output power and the compound cavity losses through the refractive index of the laser diode material. Refractive index changes result in a detuning between the externally selected wavelength and the weak internal-mode structure of the anti-reflection coated laser diode. This detuning is directly related to the compound cavity losses. On the one hand, a change in optical output power results in a change of the refractive index via the carrier density. On the other hand, it results in a change of refractive index via temperature changes. Compared to the carrier induced refractive index change, the temperature induced refractive index change is opposite in sign, a factor of ~10² smaller and slower. The switch-on and switch-off time of the self-pulsation is governed by the carrier life time. The repetition rate of the self-pulsation is governed by the thermal time constant and is in the megahertz region. Cross-modulation resulting from the thermal induced refractive index change is demonstrated. In a two-wavelength double external-cavity laser diode, optical power at one wavelength effects the optical power at the other wavelength. This cross-modulation is shown to be related to previous experiments on a laser neural network. A novel technique is introduced to measure the thermal impedance of a laser diode that is based on the cross-modulation     

Transient and optoelectronic feedback-sustained pulsation of laserdiodes at 1300 nm

We report the observation of self-sustained pulsation and transient self-pulsation in laser diodes at 1300 nm and the effects of optoelectronic feedback on the pulsations. Transient self-pulsation has a lifetime of a few minutes with frequencies up to 7 GHz. The linewidth of self-pulsation is on the order of 0.5 GHz. With optoelectronic feedback, the transient self-pulsation can be sustained and the linewidth significantly reduced to about 20 kHz. The center frequency of feedback-sustained pulsation is dependent on the passband of the bandpass filter in the feedback loop. The feedback sustained pulsation can be frequency modulated for applications in subcarrier multiplexed optical networks such as cable TV distribution and antenna remoting     

Self-pulsation in multisection laser diodes with a DFB reflector

A novel self-pulsation regime is observed in multisection laser diodes which consist of a loss-coupled distributed-feedback (DFB) section, a phase control section, and gain sections, where 10-GHz self-pulsation due to compound cavity mode beating has been reported with the DFB section operated as a single-mode laser. When the DFB section is below threshold current, the devices give the self-pulsation in a very wide operating range. We attribute the pulsation to passive mode-locking and also confirm that this structure is applicable to 40-GHz operation.     

Application of a six-port network to a near-field antenna measurement system

The configuration and calibration of a six-port network analyser for application in near-field antenna measurements is described. The suitability of the new measurement system is illustrated by experimental results.     

Radiation from linear antennas in a dissipative half-space

A formulation for the electromagnetic fields of finite linear antennas of arbitrary length immersed in a dissipative halfspace (such as the ocean or the earth) is presented. The electric fields in either medium can be readily evaluated once the current distribution is assumed or prescribed. The fields are given in each medium for each of the three major subdivisions of the horizontal range, the near-field range, the intermediate range, and the asymptotic range. Antenna patterns obtained from computer results using formulas derived in this paper are presented for some typical submerged linear antennas. These computer results are compared with experimental measurements performed with linear antennas submerged in the Atlantic Ocean.