Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for defence applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. <p> </p>We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for optical clocks and cold-atom interferometry systems. This includes the [5s<sup>2</sup>S<sub>1/2</sub>-5p<sup>2</sup>P<sub>1/2</sub>] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s<sub>2</sub><sup>1</sup>S<sub>0</sub>-5p<sup>1</sup>P1] cooling transition in neutral strontium clocks at 461 nm and the [5s<sup>2</sup><sub>s1/2</sub> – 6p<sup>2</sup>P<sub>3/2</sub>] transition in rubidium at 420 nm.<p> </p> Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.
Nitride based laser diodes utilize as an active medium extremely strained InGaN quantum wells. As the nitride materials are piezoelectric in their nature, this strain is reflected in a strong piezoelectric field. This tilts the energy bands and shifts the emission spectrum position through the Quantum Confined Stark Effect (QCSE). As the laser diode is operated at elevated currents, the built-in electric field is reduced due to the screening by injected carriers. This leads to the increase of emission energy (blue-shift). It has been a subject of many discussions whether the field is preserved at lasing, or is it completely screened.
In this work we compare the emission wavelength shift of nitride laser diodes and superluminescent diodes having different QW compositions. The superluminescent diodes allow us to study the emission spectrum at higher carrier densities than for laser diodes. In laser structures, we clearly see the saturation of the blue-shift at threshold current. While, on the other hand, we see the continuous shift of the emission wavelength in case of superluminescent diodes. This suggests, that the piezoelectric fields are not fully screened at threshold current. We also see that for UV laser diodes the emission line shift is much smaller than for blue wavelength devices. This implies practically complete screening of the electric field for UV laser while the lasing of blue laser diodes occurs at high electric field conditions.
GaN laser diodes have the potential to be a key enabling technology since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. Novel applications include high power laser bars for optical pumping, to laser sources for quantum technologies based on atom interferometry, such as next generation optical clocks and gravity sensors. <p> </p>We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for optical clocks and atom interferometry systems. This includes the [5s<sup>2</sup>S<sub>1/2</sub>-5p<sup>2</sup>P<sub>1/2</sub>] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s<sub>2</sub> <sup>1</sup>S<sub>0</sub>-5p<sup>1</sup>P<sub>1</sub>] cooling transition in neutral strontium clocks at 461 nm and the [5s<sup>2</sup>s<sub>1/2</sub> – 6p<sup>2</sup>P<sub>3/2</sub>] transition in rubidium at 420 nm. <p> </p>In addition, we report our latest results on tapered high power GaN laser diodes, for i) optical amplifiers, and ii) optimising the optical power of the device by reducing filamentation and hence avoiding catastrophic optical mirror damage (COMD).
We have realised InGaN/GaN distributed feedback laser diodes emitting at a single wavelength in the 42X nm wavelength range. Laser diodes based on Gallium Nitride (GaN) are useful devices in a wide range of applications including atomic spectroscopy, data storage and optical communications. To fully exploit some of these application areas there is a need for a GaN laser diode with high spectral purity, e.g. in atomic clocks, where a narrow line width blue laser source can be used to target the atomic cooling transition. Previously, GaN DFB lasers have been realised using buried or surface gratings. Buried gratings require complex overgrowth steps which can introduce epi-defects. Surface gratings designs, can compromise the quality of the p-type contact due to dry etch damage and are prone to increased optical losses in the grating regions. In our approach the grating is etched into the sidewall of the ridge. Advantages include a simpler fabrication route and design freedom over the grating coupling strength.Our intended application for these devices is cooling of the Sr+ ion and for this objective the laser characteristics of SMSR, linewidth, and power are critical. We investigate how these characteristics are affected by adjusting laser design parameters such as grating coupling coefficient and cavity length.
The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from UV, ~380 nm, to the visible ~530 nm, by tuning the indium content of the laser GaInN quantum well. This makes nitride laser diodes suitable for a vast range of applications, but most of them require not only the proper wavelength emission, but also high optical power and good beam quality. The typical approach - wide ridge waveguide - often suffers from spatial multimode emission (low beam quality). We report our initial results with tapered GaN lasers to increase the maximum optical power of the device with a good beam profile. This combination opens new possibilities for GaN laser diode technology in quantum technologies including optical atomic clocks and quantum gravity sensors.
GaN laser diodes has the potential to be a key enabling technology for a range of quantum technologies, including next generation optical atomic clocks and gravity sensors, based on cold-atom interferometry and also quantum communications, that have important applications for security and defence. Presently, such systems require a number of expensive, sophisticated and complex laser sources that limit quantum technologies to the laboratory. In contrast, GaN laser diode technology has the potential to provide a compact, rugged and reliable solutions, suitable for commercialisation. We report our latest results of GaN laser diodes suitable for both cold-atom interferometry and quantum communications.
GaN laser diodes fabricated from the AlGaInN material system is an emerging technology for
high power, optical integration and quantum applications. The AlGaInN material system allows
for laser diodes to be fabricated over a very wide range of wavelengths from u.v., ~380nm, to the
visible ~530nm, by tuning the indium content of the laser GaInN quantum well, giving rise to
new and novel applications including displays and imaging systems, free-space and underwater
telecommunications and the latest quantum technologies such as optical atomic clocks and atom
Optical clocks have demonstrated an improvement in temporal accuracy of several orders of magnitude over existing time standards based on caesium. Such systems hold great promise in many industrial sectors including financial time stamping, GPS-free navigation and network synchronisation. Atom interferometry has proven to be a reliable method of precision gravity sensing and finds application in geological studies, including earthquake warning systems and oil exploration. Such systems require a number of sophisticated lasers in a compact and reliable format for use outside of a laboratory environment, suitable for commercialisation and user transportation. Of particular interest, is emerging AlGaInN laser diode technology that has the potential to provide practical solutions for next generation optical clock technology.
In this paper, an impact of mounting of structures of nitride laser bars their performance, emitted optical power in particular, is presented. The laser bars of nitride edge-emitting lasers of ridge-waveguide type the InGaN/GaN active areas have been considered. Laser performance has been analysed with the aid of an advanced self-consistent thermalelectrical model, calibrated using experimental data for a single diode laser. The simulated laser bars emit at 408 nm. An optimal number of laser emitters and their various arrangments have been considered. An appliation of Cu heat sinks of various dimensions as well as the p-side-up or the p-side-down laser configurations have been analysed. Moreover a possible application of a diamond heat spreader has been also taken into account.
The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v., ~380nm, to the visible ~530nm, by tuning the indium content of the laser GaInN quantum well. Thus AlGaInN laser diode technology is a key enabler for the development of new disruptive system level applications in displays, telecom, defence and other industries.
We have developed highly compact RGB laser light modules to be used as light sources in multi-view autostereoscopic
outdoor displays and projection devices. Each light module consists of an AlGaInP red laser diode, a GaInN blue laser
diode, a GaInN green laser diode, as well as a common cylindrical microlens. The plano-convex microlens is a so-called
“fast axis collimator”, which is widely used for collimating light beams emitted from high-power laser diode bars, and
has been optimized for polychromatic RGB laser diodes. The three light beams emitted from the red, green, and blue
laser diodes are collimated in only one transverse direction, the so-called “fast axis”, and in the orthogonal direction, the
so-called “slow axis”, the beams pass the microlens uncollimated. In the far field of the integrated RGB light module this
produces Gaussian beams with a large ellipticity which are required, e.g., for the application in autostereoscopic outdoor
displays. For this application only very low optical output powers of a few milliwatts per laser diode are required and
therefore we have developed tailored low-power laser diode chips with short cavity lengths of 250 μm for red and
300 μm for blue. Our RGB laser light module including the three laser diode chips, associated monitor photodiodes, the
common microlens, as well as the hermetically sealed package has a total volume of only 0.45 cm³, which to our
knowledge is the smallest RGB laser light source to date.
The gain saturation is a crucial factor limiting achievable output power of superluminescent diodes (SLD), as it exponentially depends on optical gain value. Contrary to laser diodes, in SLDs gain is increasing with the increasing current even much above the transparency conditions. Therefore, SLDs provide us with an unique possibility to examine gain under high current densities (high carrier injection). In our work we examined SLDs fabricated in a “j-shape” ridge-waveguide geometry having chips of the length of 700 μm and 1000 μm, emitting in the blue-violet region. By comparing the amplified spontaneous emission measured along the device waveguide with true spontaneous emission measured in perpendicular direction, we are able to extract optical gain as a function of injected current. We show, that in our devices spontaneous emission exhibits a square-root-like dependence on current which is commonly associated with the presence of “droop” in case of nitride light emitting diodes. However, along the waveguide axis, fast processes of stimulated recombination dominate which eliminates the efficiency reduction. Calculated optical gain shows a substantial saturation for current densities above 8 kA/cm<sup>2</sup>.
The latest developments in AlGaInN laser diode technology are reviewed. The AlGaInN material
system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v. to
the visible, i.e., 380-530nm, by tuning the indium content of the laser GaInN quantum well. Of
specific interest for defence applications is blue-green laser diode technology for underwater
telecommunications and sensing applications.
Ridge waveguide laser diode structures are fabricated to achieve single mode operation with optical
powers of <100mW in the 400-420nm wavelength range with high reliability. Low defectivity and
highly uniform GaN-substrates allow arrays and bars of nitride lasers to be fabricated. In addition,
high power operation of AlGaInN laser diodes is demonstrated with the operation of a single chip,
‘mini-array’ consisting of a 3 stripe common p-contact at powers up to 2.5W cw in the 408-412 nm
wavelength range and a 16 stripe common p-contact laser array at powers over 4W cw.
Junction temperature of a laser diode (LD) determines the value of threshold current, maximum achievable power and device lifetime. In this work we studied this parameter by a method of comparing current-voltage characteristics measured under pulse bias (at various temperatures) with DC characteristic obtained at room temperature. As exemplary devices we chose various laser diode arrays and single emitter laser with different substrate thickness. The results show, that the primary factor determining thermal resistance of the device is the chip’s surface, which means, that a dominating mechanism is related with a heat transfer between the chip and the heat sink.
We present ultraviolet InGaN superluminescent diodes fabricated in a “j-shape” waveguide geometry. Under CW operation at room temperatures, devices emit optical power up to 80 mW at 395 nm with no tendency for lasing. The chip length was 1.5 mm. Emitted optical power was very sensitive to the device temperature. This effect limited the maximum optical power obtained in CW operation. With better packaging scheme better performance in CW regime should be achieved.
We demonstrate the possibility of fabrication of InGaN laser diode with an extremely thin lower AlGaN cladding (200 nm) by using high electron concentration, plasmonic GaN substrate. The plasmonic substrates were fabricated by one of high-pressure methods – ammonothermal method or multi-feed-seed growth method and have an electron concentration from 5x10<sup>19</sup> cm<sup>-3</sup> up to 1x10<sup>20</sup> cm<sup>-3</sup>. New plasmonic substrate devices, in spite of their extremely thin lower AlGaN cladding, showed identical properties to these manufactured with traditional, thick lower cladding design. They were characterized by identical threshold current density, slope efficiency and differential gain. Thin AlGaN devices are additionally characterized by low wafer bow and very low density of dislocations (<10<sup>4</sup> cm<sup>-2</sup>).
Highly n-doped GaN is a material of a reduced refractive index which may substitute AlGaN as a cladding layer in
InGaN laser diodes. In this study we focus on the determination of the optical absorption and the refractive index of
GaN:O having the electron concentration between 1·10<sup>18</sup> - 8·10<sup>19</sup> cm<sup>-3</sup>. Though the measured absorption coefficient for
the highest doped GaN are rather high (200 cm<sup>-1</sup>) we show, using an optical mode simulation, that you can design a
InGaN laser diode operating in blue/green region with decent properties and low optical losses. We propose to use
relatively thin AlGaN interlayer to separate plasmonic GaN from the waveguide and thus to dramatically reduce the