Birgitte Thestrup

Design and Simulation of Next-Generation High-Power, High-Brightness Laser Diodes

High-brightness laser diode technology is progressing rapidly in response to competitive and evolving markets. The large volume resonators required for high-power, high-brightness operation makes their beam parameters and brightness sensitive to thermal- and carrier-induced lensing and also to multimode operation. Power and beam quality are no longer the only concerns for the design of high-brightness lasers. The increased demand for these technologies is accompanied by new performance requirements, including a wider range of wavelengths, direct electrical modulation, spectral purity and stability, and phase-locking techniques for coherent beam combining. This paper explores some of the next-generation technologies being pursued, while illustrating the growing importance of simulation and design tools. The paper begins by investigating the brightness limitations of broad-area laser diodes, including the use of asymmetric feedback to improve the modal discrimination. Next, tapered lasers are considered, with an emphasis on emerging device technologies for applications requiring electrical modulation and high spectral brightness.

High brightness laser source based on polarization coupling of two diode lasers with asymmetric feedback

In this letter, we show that polarization coupling and asymmetric diode-laser feedback can be used to combine two diode-laser beams with low spatial coherence into a single beam with high spatial coherence. The coupled laser source is based on two similar laser systems each consisting of a 1 μm×200 μm broad area laser diode applied with a specially designed feedback circuit. When operating at two times threshold, 50% of the freely running system output power is obtained in a single beam with an M2 beam quality factor of 1.6±0.1, whereas the M2 values of the two freely running diode lasers are 29±1 and 34±1, respectively.

Electrical and optical properties of thin indium tin oxide films produced by pulsed laser ablation in oxygen or rare gas atmospheres

Films of indium tin oxide (ITO) have been produced in different background gases by pulsed laser deposition (PLD). The films deposited in rare gas atmospheres on room temperature substrates were metallic, electrically conductive, but had poor transmission of visible light. For substrate temperatures at 200°C, the specific resistivity was reduced and the transmission of visible light enhanced for all background gases. Films produced in oxygen turned out to be superior to films deposited in other gases at the same temperature.

Self-injection locking of an extraordinarily wide broad-area diode laser with a 1000-μm-wide emitter

The experimental results of self-injection locking of an antireflection-coated broad-area diode laser with a 1000-microm-wide emitting area are presented. To our knowledge, it is the broadest single-element diode laser that has been used in an external-feedback cavity until now. Usually, wide diode lasers suffer from filamentation, which leads to poor spatial beam quality. We show, however, that the beam quality of the diode laser is improved significantly when we use asymmetric self-injection locking. An output power of 2.05 W is obtained with a beam quality factor M2 of 2.7. The self-injection locking technique improves the beam quality by a factor of 107. By comparing the results with those obtained with an ordinarily coated diode laser with a 1000-microm-wide emitter we show that antireflection coating on the front facet is decisive for this improvement in the beam quality.

Spectral beam combining of a 980 nm tapered diode laser bar

We demonstrate spectral beam combining of a 980 nm tapered diode laser bar. The combined beam from 12 tapered emitters on the bar yielded an output power of 9.3 W at 30 A of operating current. An M2 value of 5.3 has been achieved along the slow axis. This value is close to that of a free running single tapered emitter on the bar at the same current level. The overall spectral beam combining efficiency was measured to be 63%.

980 nm high brightness external cavity broad area diode laser bar

We demonstrate off-axis spectral beam combining applied to a 980 nm high power broad area diode laser bar. The experiments yielded 9 W of optical power at 30 A of operating current and the measured M2 values of the combined beam from 12 emitters were 1.9 and 6.4 for the fast and the slow axis, respectively. The slow axis beam quality was 5-6 times better than the value obtained from a single emitter in free running mode. A high brightness of 79 MW/cm2-str was achieved using this configuration. To our knowledge, this is the highest brightness level ever achieved from a broad area diode laser bar.

Improvement of the beam quality of a broad-area diode laser using double feedback from two external mirrors

In this letter, a symmetric double-feedback configuration, to improve the beam quality of broad-area diode lasers is demonstrated. With this configuration, a symmetric double-lobed far field can be obtained, and this configuration leads to good beam quality. The beam quality factor M2 of a diode laser with the emitting area 1μm×200μm is improved by using both the asymmetric single feedback and the symmetric double feedback. M2 values of 4.3 for the asymmetric single-feedback laser system and 3.3 for the symmetric double-feedback laser system are obtained, whereas the M2 value of the freely running laser is 42. The far and the near fields are also measured and compared for the three conditions.

Narrow line width operation of a 980 nm gain guided tapered diode laser bar

We demonstrate two different schemes for the spectral narrowing of a 12 emitter 980 nm gain guided tapered diode laser bar. In the first scheme, a reflective grating has been used in a Littman Metcalf configuration and the wavelength of the laser emission could be narrowed down from more than 5.5 nm in the free running mode to 0.04 nm (FWHM) at an operating current of 30 A with an output power of 8 W. The spectrum was found to be tunable within a range of 16 nm. In the second scheme, a volume Bragg grating has been used to narrow the wavelength of the laser bar from over 5 nm to less than 0.2 nm with an output of 5 W at 20 A. To our knowledge, this is the first time spectral narrowing has been performed on a gain guided tapered diode laser bar. In the Littman Metcalf configuration, the spectral brightness has been increased by 86 times and in the volume Bragg grating cavity the spectral brightness has been improved over 18 times when compared to the free running operation. These schemes could be also extended for other wavelengths of interest in the future.

Lateral mode selection in a broad-area laser diode by self-injection locking with a mirror stripe

In this paper, we demonstrate lateral mode selection and amplification in a broad area laser (BAL) diode in an external cavity. The cavity is based on self-injection locking of an 807 nm, 3W broad area diode using a mirror stripe as the feedback unit. At the optimum mirror stripe position, the lateral far-field intensity profile is narrowed 8.5 times compared with the profile from the freely running laser when running at a drive current of twice the threshold current. We have determined the lateral angular range, in which, different array modes can be exited and, only, within a narrow range around 2.3° from the beam center a high, spatial beam coherence can be obtained.

Ablation from metals induced by visible and UV laser irradiation

The deposition rate of laser-ablated silver has been determined for fluences between 0.5 and 15 J / cm 2 at the wavelengths 532 and 355 nm for a beam spot area of around 0.01 cm 2 . The ablated metal was collected on a quartz crystal microbalance. The rate at 5 J / cm 2 was about 4 x 10 13 Ag / cm 2 per pulse for 532 nm, and somewhat lower for 355 nm. The initial vaporization during the ablation has been studied numerically as well.