L. Goldberg

Single-mode operation of a coiled multimode fiber amplifier

We report a new approach to obtaining single-transverse-mode operation of a multimode fiber amplifier in which the gain fiber is coiled to induce significant bend loss for all but the lowest-order mode. We demonstrated this method by constructing a coiled amplifier using Yb-doped, double-clad fiber with a core diameter of 25 microm and a numerical aperture of ~0.1 (V approximately 7.4) . When the amplifier was operated as an amplified-spontaneous-emission source, the output beam had an M(2) value of 1.09 +/- 0.09 ; when seeded at 1064 nm, the slope efficiency was similar to that of an uncoiled amplifier. This technique will permit scaling of pulsed fiber lasers and amplifiers to significantly higher pulse energies and peak powers and cw fiber sources to higher average powers while maintaining excellent beam quality.

Spectral characteristics of semiconductor lasers with optical feedback

Optical feedback-induced changes in the output spectra of several GaAlAs lasers operating at 0.83 μm are described. The feedback radiation obtained from a mirror 60 cm away from the laser is controlled in intensity and phase. Spectral line narrowing or broadening is observed in each laser depending on the feedback conditions. Minimum linewidths observed with feedback are less than 100 kHz. Improved wavelength stability is also obtained with optical feedback resulting in 15 dB less phase noise. An analytical model for the three-mirror cavity is developed to explain these observations.

Fiber-optic prism true time-delay antenna feed

Experimental results for an optical-control technique for implementing a true time-delay function for array antennas are reported. A microwave signal is transmitted on a wavelength-tunable optical carrier through a fiber-optic prism-a set of nominally equal-delay fibers with differing net dispersion-to photodetectors that feed each antenna element. The relative interelement time-delay (beam angle) adjustment is accomplished by tuning the optical carrier wavelength. Measured antenna patterns of a two-element array clearly demonstrate beam steering and true time-delay operation over a two-octave bandwidth of 2-8 GHz.<<ETX>>

Injection locking of coupled‐stripe diode laser arrays

The control of the far‐field beam pattern and the spectrum of 10‐element laser diode array by injection locking to a single‐mode master laser are described. With less than 3 mW of injected power an array output of 105 mW at a single frequency with a 0.5° wide far‐field lobe is obtained. Similar results are observed when the entire array is illuminated by the master laser beam, or when only one of the stripes is illuminated. The narrow lobe contains 60–70% of the total power and is centered at an angle of about 4° relative to the normal of the array facet.

Second‐harmonic generation at 421 nm using injection‐locked GaAlAs laser array and KNbO3

Significant improvement in frequency doubling efficiency of a cw output of a GaAlAs laser diode is described. Up to 0.72 mW of 421 nm power was generated by illuminating a KNbO3 crystal with a 270 mW diffraction‐limited beam generated by an externally injection‐locked laser diode array, operating in a single‐mode and single‐far‐field lobe.

Ultrawide tunable Er soliton fiber laser amplified in Yb-doped fiber.

A Raman-shifted and frequency-doubled high-power Er-fiber soliton laser for seeding an efficient high-power Yb fiber femtosecond amplifier is demonstrated. The Raman-shifted and frequency-doubled Er-soliton laser is tunable from 1.00 to 1.070microm and produces bandwidth-limited 24-pJ pulses at a repetition rate of 50 MHz with a FWHM pulse width of 170 fs at 1.040microm . The Yb(3+) amplifier has a slope efficiency of 52% and generates 3-ps linearly chirped pulses with an average power of 0.8 W at 1.05microm . After pulse compression, 74-fs bandwidth-limited pulses with an average power of 0.4 W and a pulse energy of 8 nJ are generated.

Technique for lateral temperature profiling in optoelectronic devices using a photoluminescence microprobe

A photoluminescence microprobe technique with ≤0.2 °C and <5 μm resolution is demonstrated for measuring lateral temperature profiles in GaAs‐based optoelectronic devices. The technique is used to measure the junction‐heating induced temperature gradient in both single‐stripe and broad‐area diode lasers. The effective focal length of the thermally induced refractive index gradient lens is determined from the temperature gradient in a broad‐area device.

5.25-W CW near-diffraction-limited tapered-stripe semiconductor optical amplifier

A broad-area tapered-contact single-pass amplifier emitting at 860-nm wavelength is demonstrated to emit up to 5.25 W continuous wave (CW) in a near-diffraction-limited radiation pattern. The diffraction-limited component of the radiation pattern, comprising greater than 87% of the total power at 5.25-W CW output, is observed to decrease slightly with increasing drive current due to filament formation. The output beam astigmatism is found to saturate at high power output in accordance with gain saturation, which indicates that the high-quality output beam remains stable with respect to small changes in current or injected power.<<ETX>>

Second harmonic generation in field poled, quasi-phase-matched, bulk LiNbO/sub 3/

Field induced periodic poling is used to create >10 micron period structures in /spl sim/250 micron thick slabs of LiNbO/sub 3/. Third order quasi-phase-matched second harmonic blue light generation is demonstrated by free propagation of a 937 nm fundamental Gaussian beam in periodically poled LiNbO/sub 3/, with a conversion efficiency of 0.2%/W cm.<<ETX>>

High-power, near-diffraction-limited large-area traveling-wave semiconductor amplifiers

Operating characteristics of high-power large-active-area GaAlAs amplifiers configured in double-pass and single-pass traveling-wave arrangements are described. Single-pass broad-area amplifiers with a 600- mu m stripe width generated up to 21 W of near-diffraction-limited emission under pulsed operation when injected with 500 mW from a Ti:Al/sub 2/O/sub 3/ laser, and 11.6 W when injected with 100 mW from a laser diode master laser. In CW operation, a broad-area amplifier output of 3.3 W was demonstrated. Tapered-stripe large-area amplifiers emitted up to 4.5 W in a near-diffraction-limited beam when injected with 150 mW from a Ti:Al/sub 2/O/sub 3/ laser. The physical mechanisms causing degradation of the output beam phase front and intensity uniformity at high output power levels, including thermal lensing and filamentation, are presented. >