摘要:

Laser‐induced stresses resulting from high‐power laser‐material interactions have been studied extensively. However, the rate of change in mechanical energy, or stress power, due to laser‐induced stresses has only recently been investigated. An unanswered question for monitoring laser‐material interactions in the far‐field is whether stress power differs from stresses measured, particularly with respect to laser‐energy coupling to a solid target. This letter shows experimental acoustic data which demonstrate that stress power measured in the far field of the target shows changes in laser‐energy coupling, whereas the stresses measured do not. For the ambient medium above the target, stress power and stress together reflect changes in laser‐energy coupling. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114880

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

For lasers modulated with a pseudorandom bit stream without prebias, the delay between the optical pulse and the current pulse depends on the number of ‘‘0’’s preceding the current pulse (‘‘1’’). Since 1’s and 0’s occur randomly in a pseudorandom bit stream and the decision circuit is designed to start sampling at a given time, this effect results in a timing jitter. The jitter induced noise and the associated power penalty are calculated. The results suggest that the ratio of the modulation current to the threshold current of the laser should be ≳25 to keep the power penalty below 1 dB at high data rates. Low threshold lasers operating without prebias are important for laser array based transmitters. ©1995 American Institute of Physics. 

ISSN:0003-6951    

DOI:10.1063/1.114881

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We demonstrate an undoped 20 1/2 pair AlAsSb/GaAsSb distributed Bragg reflector (DBR) grown lattice matched to an InP substrate by molecular beam epitaxy. Reflectivity measurements indicate a stop band centered at 1.78 &mgr;m with a maximum reflectivity exceeding 99%. We also measure current–voltage characteristics in a similar 10 1/2 periodp‐type DBR and find that a current density of 1kA/cm2produces a 2.5 V drop. Hole mobilities and doping concentrations in AlAsSb and GaAsSb are also reported. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114882

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In this letter generation of up to 130 GHz high frequency self‐pulsation in ultrafast gain‐switched InGaAs/InGaAsP dynamical single‐mode lasers with monolithically integrated external cavities is reported. With various active cavity lengths and external cavity lengths, the dependence of the pulsation frequency on the length of the respective cavity is shown and analyzed in detail for a pulsation frequency range from 50 to 130 GHz. Theoretically, by treating the external cavity as a gain‐switched laser amplifier and taking into account the coherent time‐dependent amplification, shortening and reflection of an incident picosecond optical pulse in the gain‐switched amplifier, we can well understand and simulate the observed high frequency self‐pulsations. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114883

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Tunable second harmonic radiation has been generated on a gold surface by means of a dye laser system capable of emitting pulses of 450 fs and 4.5 ps duration. The intensity of harmonic radiation was maximum along the direction of the reflected incident laser beam, when the polarization direction of the incoming radiation was vertical to the target surface (ppolarization). The efficiency for second harmonic generation was found to depend strongly on the pulse duration of the laser beam. The observed change of the harmonic generation efficiency as a function of the angle of incidence and the pulse duration of the laser beam suggests that, due to coherence effects, the actual overlapping spatial dimension of the interaction region between the laser pulse and the electron gas of the metallic surface plays an important role. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114884

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We experimentally observed the self‐channeling of the light beam in photorefractive Bi12TiO20fiberlike sample owing to the strong fanning effect. It results in significant speed up of the photorefractive response time in waveguides and in anomalous dependence of the response time on the applied ac electric field amplitude: the higher electric field, the faster response. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114885

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

43 GHz repetition rate Fourier transform‐limited light pulses of a width of 1.5 ps are achieved by passively mode‐locking an ion‐implanted semiconductor laser in a very short external cavity. A higher repetition rate is limited by strongly mixed self‐pulsation. A numerical investigation of passive model‐locking is carried out based on the solution of traveling wave rate equations of an external cavity semiconductor laser. We find that the gain nonlinearity factor can significantly influence the present range of the mixed self‐pulsation. With the increase of the nonlinearity factor, the operation tends to pure passive mode‐locking at a low injection current. In addition, the proper adjustment of the ratio of differential absorption to differential gain and the carrier lifetime in the absorber can reduce the necessary current for pure passive mode‐locking. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114886

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A method for measuring surface displacements is described. The principle is based on the detection of the phase shift of a high frequency continuous ultrasonic wave reflected from the moving surface. The analysis shows that the long time delay undergone by the ultrasonic wave produces a significant enhancement of the Doppler phase shift signal. Experiments were carried out in water with a 30‐MHz focused transducer probe and transient mechanical displacements smaller than 1 A˚ (surface velocities smaller than 1 mm/s) were detected in a 10 MHz bandwidth. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114887

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We present an improved version of the classical heterodyne interferometer for ultrasound detection, which includes a double phase conjugate mirror (DPCM) in the detection arm. The DPCM performs wave front adaptation to plane wave of the speckled beam scattered by the rough surface of the sample. The performances of the system are analyzed regarding the transmission (≊30%) and fidelity (≊70%) of the phase conjugation and the response time and e´tendue of the device. An example of application to the detection of laser generated ultrasound is also presented.

ISSN:0003-6951    

DOI:10.1063/1.114888

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Multiple beams in a heavy ion fusion reactor chamber can increase the beam spot size at the target. Each beam experiences the field from nearby beams and this field varies along the bunch length. These field variations cause different parts of the beam to be focused at different locations and results in a larger spot size at the target. Calculations show that this is not a large effect for a beam generated by an induction linear accelerator in which the current is nearly constant over much of the pulse. These calculations show 90%–99% of the charge reaches the target for a radiation converter radius that is 10%–20% larger than the beam radius. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114889

出版商:AIP    

年代:1995

数据来源: AIP