Tapered bow-tie lasers for high energy Q-switched pulse generation

A new kind of Q switched laser, the bow tie laser is introduced. This type of laser permits large area facets at both ends so that generation of high optical powers involve low optical intensities to prevent optical damage. The incorporation of doubled tapered waveguide structure to the Q switched multicontact laser has increased the optical pulse energies and peak powers of the laser.

Bias-dependent recovery time of all-optical resonant nonlinearity in InGaAsP/InGaAsP MQW waveguide

We have investigated a resonant refractive nonlinearity in a semiconductor waveguide by measuring intensity dependent phase shifts and bias-dependent recovery times. The measurements were performed on an optimized 750-μm-long AR coated buried heterostructure MQW p-i-n waveguide with a bandedge at 1.48 μm. Figure 1 shows the experimental arrangement. The mode-locked color center laser was tuned to 50 meV beyond the bandedge and 8 ps pulses with peak incident power up to 57 W were coupled into the waveguide. Some residual bandtail absorption remains at this wavelength and this is sufficient to cause carriers to be photogenerated and these give rise to a refractive nonlinearity, predominantly by plasma and bandfilling effects. A Fabry-Perot interferometer is used to measure the spectrum of the light which exits the waveguide. The nonlinearity within the guide causes self phase modulation (SPM) of the light and a study of the spectrum allows information to be recovered on the magnitude and recovery time of the nonlinear phase shift with a reasonable degree of accuracy. SPM spectra were recorded for a variety of pulse energies coupled into he unbiased waveguide. Figure 2 shows the resultant phase shift measured from the SPM spectra as a function of pulse energy. The relationship is a linear one, indicating that no saturation of the nonlinearity occurs for coupled pulse energies up to 230 pJ. A π phase shift, the minimum necessary for an all-optical switch, is obtained for a coupled pulse energy of 57 pJ while the maximum phase shift, 4 π, was measured for 230 pJ. The SPM spectra were highly asymmetric with pulse energy shifted to higher frequencies. Such spectra are characteristic of a slow, negative nonlinearity. This relatively slow speed is expected for the unbiased guide as the recovery time will be of the order of the recombination time of the photogenerated electrons, about 1 ns for InGaAsP material. In order to reduce the recovery time of the nonlinearity, it is necessary to remove the photogenerated carriers from the waveguide by a process other than recombination. One such technique is to apply a reverse bias to the waveguide in order to sweep the carriers out. Figure 3 shows the effect on the recovery time of the nonlinearity of applying reverse bias to the waveguide for 230 pJ coupled power. The recovery time was reduced from one much longer than the length of the pulse, estimated to be about 1 ns, at zero bias to 18 ± 3 ps for a bias voltage greater than -4 V. This compares with a value of 24 ps obtained in a bulk waveguide.

Demonstration of multi wavelength picosecond pulse generation with resonant RF drive using an monolithically integrated multi-grating cavity (MGC) laser

Multiwavelength pulses were generated using a monolithically integrated device. The device used is an integrated InGaAs/InGaAsP/InP multi-wavelength laser fabricated by selective area regrowth. The device self pulsated on all of the four wavelength channels. 48 ps pulses were obtained which were measured by a 50GHz oscilloscope and 32GHz photodiode which was not bandwidth limited. Simultaneous multi-wavelength pulse generation was also achieved.

Nonlinear pulse switching and compression in a dispersion-imbalanced loop mirror

The results of the high-quality nonlinear pulse compression of gain-switched laser diode pulses using a two-cascade compression scheme are presented. The scheme incorporates a dispersive delay line and a nonlinear pulse compressor based on a dispersion-imbalanced fiber loop mirror (DILM). It is demonstrated that the DILM can be also used for the pulse compression with a compression ratio of 10 or higher.

Multi-wavelength picosecond pulse generation using a monolithically integrated Multi-wavelength Grating Cavity (MGC) laser

Multi-wavelength picosecond pulses are demonstrated using a single monolithically integrated Multi-wavelength Grating Cavity (MGC) laser. This is achieved on two WDM wavelength channels at a repetition rate of 7.63 GHz.

Picosecond pulse source for OTDM/WDM applications based on arrayed waveguide grating

A novel technique for high quality femtosecond pulse generation from a gain-switched laser diode by means of pulse compression and transformation in a compact nonlinear fiber device, based on a dispersion-imbalanced fiber loop mirror (DILM) is demonstrated. This source allows the generation of extremely high quality pulses as short as 270 fs on demand with strong suppression of pulse pedestals. Spectral filtering in arrayed waveguide grating (AWG) converts the device into a compact multiwavelength source of high-quality picosecond pulses for optical time division multiplexing/wavelength division multiplexing applications.

High-power and picosecond pulse generation from a passively Q-switched tapered InGaAs/GaAs laser

Tapered waveguides have been used for enhancing pulse powers in Q-switched AlGaAs and InGaAsP lasers. This paper reports on passively Q-switched pulses with 1.53 W peak power and 41-ps FWHM from an InGaAs/GasAs (970 nm) double-contact tapered semiconductor laser in a well defined single-lobed far-field.

Numerical analysis of the coupled circuit and cooling holes for an electromagnetic shaker

This paper presents a time-stepping shaker modeling scheme. The new method improves the accuracy of analysis of armature-position-dependent inductances and force factors, analysis of axial variation of current density in copper plates (short-circuited turns), and analysis of cooling holes in the magnetic circuit. Linear movement modeling allows armature position to be precisely included in the shaker analysis. A more accurate calculation of eddy currents in the coupled circuit is in particular crucial for the shaker analysis in a mid-or high-frequency operation range. Large currents in a shaker, including eddy currents, incur large Joule losses, which in turn require the use of a cooling system to keep temperature at bay. Sizable cooling holes have influence on the saturation state of iron poles, and hence have to be properly taken into account.