David Paboeuf

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.

Narrow-line coherently combined tapered laser diodes in a Talbot external cavity with a volume Bragg grating

We present the phase locking of an array of index-guided tapered laser diodes. An external cavity based on the self-imaging Talbot effect has been built. A volume Bragg grating is used as the output coupler to stabilize and narrow the spectrum at 976nm. A power of 1.7W is achieved in the in-phase single main lobe mode with a high visibility. We have checked that each emitter is locked to the Bragg wavelength with a 100pm spectrum linewidth. The experimental results compare well with numerical simulations performed with two-dimensional wide-angle finite difference beam propagation method.

Frequency stabilization at the kilohertz level of a continuous intracavity frequency-doubled singly resonant optical parametric oscillator

A continuous intracavity frequency-doubled singly resonant optical parametric oscillator (OPO) is stabilized to the side of the transmission peak of a medium finesse Fabry-Perot cavity. The narrow bandwidth of the frequency noise of this OPO allows this simple scheme to lead to a stability of a few kilohertz with respect to the locking etalon. The system, operating in the visible domain, remains locked for more than 1 h.

Red and orange laser operation of Pr:KYF 4 pumped by a Nd:YAG/LBO laser at 469.1nm and a InGaN laser diode at 444nm

We report the basic luminescence properties and the continuous-wave (CW) laser operation of a Pr(3+)-doped KYF(4) single crystal in the Red and Orange spectral regions by using a new pumping scheme. The pump source is an especially developed, compact, slightly tunable and intra-cavity frequency-doubled diode-pumped Nd:YAG laser delivering a CW output power up to about 1.4 W around 469.1 nm. At this pump wavelength, red and orange laser emissions are obtained at about 642.3 and 605.5 nm, with maximum output powers of 11.3 and 1 mW and associated slope efficiencies of 9.3% and 3.4%, with respect to absorbed pump powers, respectively. For comparison, the Pr:KYF(4) crystal is also pumped by a InGaN blue laser diode operating around 444 nm. In this case, the same red and orange lasers are obtained, but with maximum output powers of 7.8 and 2 mW and the associated slope efficiencies of 7 and 5.8%, respectively. Wavelength tuning for the two lasers is demonstrated by slightly tilting the crystal. Orange laser operation and laser wavelength tuning are reported for the first time.

Coherent beam superposition of ten diode lasers with a Dammann grating

We demonstrate the use of a binary diffractive optical element in a very simple setup to convert the multilobed beam from a low fill factor array of coherent laser diodes into a quasi-Gaussian beam. The phase profile of the grating is determined with a phase retrieval algorithm. Experimentally, the conversion efficiency reaches more than 44%. We also establish that this setup can be used to make an effective measurement of the coherency of the laser array.

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.

Stimulated optical pumping in a Tm3+:YAG crystal

The possibility of using stimulated emission to improve the optical pumping of Tm3+ ions embedded in Tm3+:YAG to the 3F4 level is explored. The oscillator strengths and the frequencies of the transitions between the ground crystal field level of 3H4 to the different crystal field levels of 3F4 are measured at low temperature. Stimulated emission measurements at 1461 nm are performed to evaluate the optical pumping efficiency. Application to the spectral tailoring of Tm3+:YAG for quantum information applications is discussed.

Quasi-diffraction limited emission from an array of tapered laser diodes in volume Bragg grating external cavities

High-brightness single laser diodes based on the widespread taper design have demonstrated output powers of a few Watts with a single transverse mode operation [1]. The use of arrays of such lasers result in a further increase of the laser power, but with the drawback of a loss in the spatial brightness. To overcome this limitation numerous external-cavity configurations have been proposed which induce a coherence between the individual emitters of the array and result in a brightness improvement [2]. In this work we describe two external cavities intended to improve the spatial brightness of a bar of N = 6 index-guided tapered laser diodes emitting around 975 nm. The lateral structure of the emitters consists of a short ridge single-mode section, a 2.3 mm-long narrow-angle tapered ridge and a common amplified free-space 0.2 mm-long section. The array pitch is p = 30 µm, and the near-field 1/e2 full-width (1/e2-FW) of each emitter is 30 µm too, so the filling factor of this array is 100% on the front facet and the emission section is w = 180 µm wide. No adjacent coupling between emitters is evidenced in the free running laser emission of the array alone, and its 1/e2-FW divergence is ∼80 mrad in the slow axis. Our external cavity designs aim at controlling the slow-axis beam divergence of the whole array by inducing an angular-filtered feedback into the lasers [3,4]. The configuration forces the array to operate in the out-of-phase mode, which has two main lobes in its far-field profile at ± λ/2p = ± 16 mrad. We take benefit of the angular selectivity of volume Bragg gratings to favour an asymmetrical feedback on one of these peaks. The far-field of the extended-cavity array is thus expected to exhibit one diffraction-limited peak in the symmetric direction (Figure 1). Two different setups have been investigated experimentally:

External-cavity designs for phase-coupled laser diode arrays

Both configurations have been extensively studied theoretically as well as experimentally. We have obtained a stable coherent operation of the laser arrays, on a single transverse array mode. The main advantage of the angular filtering cavity is here that it is perfectly adapted to the high filling-factor laser bar, and then provides directly a nearly diffraction-limited output beam; nevertheless these bars have limited output powers because of the strong heating of the array. On the other hand, Talbot external cavities may be used with very different kind of bars, in simple extended-cavity designs. Finally, the use of the filtering properties of VBG has been demonstrated to efficiently control the laser emission.

The UK National Quantum Technology Hub in Sensors and Metrology

The UK National Quantum Technology Hub in Sensors and Metrology is one of four flagship initiatives in the UK National of Quantum Technology Program. As part of a 20-year vision it translates laboratory demonstrations to deployable practical devices, with game-changing miniaturized components and prototypes that transform the state-of-the-art for quantum sensors and metrology. It brings together experts from the Universities of Birmingham, Glasgow, Nottingham, Southampton, Strathclyde and Sussex, NPL and currently links to over 15 leading international academic institutions and over 70 companies to build the supply chains and routes to market needed to bring 10–1000x improvements in sensing applications. It seeks, and is open to, additional partners for new application development and creates a point of easy open access to the facilities and supply chains that it stimulates or nurtures.