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In this work, we present the development of low consumption quantum cascade lasers across the mid-IR range. In particular, short cavity single-mode lasers with optimised facet reflectivities have been fabricated from 4.5 to 9.2 μm. Threshold dissipated powers as low as 0.5 W were obtained in continuous wave operation at room temperature. In addition, the beneficial impact of reducing chip length on laser mounting yield is discussed. High power single-mode lasers from the same processed wafers are also presented.
© 2015 Optical Society of America
1. Introduction
In the recent years, significant improvement has been made in the development of efficient high power quantum cascade lasers [1]. This effort has led to multi-watt emission from quantum-cascade lasers (QCL) using devices that can consume up to 20–50 W of electrical power [2]. These devices have an important impact on many applications, e.g. counter-measures, photoacoustic spectroscopy or cavity ring-down spectroscopy. However, in many of the commercial applications, optical powers of few tens of mWs are sufficient. The minimization of the power dissipation in the light source becomes crucial [3–5]. In fact, in order to remove the heat generated by a standard QCL, the power consumption of the cooling system itself can exceed 100W, constraining heavily the packaging options and domains of application of these sources.
In the present work, we will focus on the development of low consumption single mode QCLs from 4.5 μm to 9.2 μm. In addition, we analyze on the impact of the chip length on the device’s price and fabrication yield. Optimizing the chip length and the facet reflectivity rather than active region or grating design, allows moreover to fabricate both low dissipation devices and high optical power devices at the same time.
2. Chip yield
The major cause of failure in QCLs is related to the presence of defects along the laser waveguide. These defects can be originated both from the epitaxial growths and from the laser device fabrication steps. In order to reduce the defect density, fabrication process optimization was carried out in order to produce buried heterostructure devices with narrow ridges while keeping low optical losses. The active region etching procedure was optimized resulting in lasers with active regions as narrow as 2.5 μm, e.g. see Fig. 1. Only wet etching has been used in order to minimize the sidewalls roughness. The lasers were then processed in a buried heterostructure configuration using Metalorganic Vapour Phase Epitaxy (MOVPE) for the selective growth of Iron-doped InP. In case of randomly distributed defects, the failure probability follows a Poissonian law which depends both on the defect density λ and on the device dimensions:
Based on the data collected by our team in the previous years, we were able to define an effective defect density of 4 e − 5 defects/mm2 which results in an approximate failure rate of 25 % for a 2.5 mm-long laser. The absolute values are still preliminary and based only on local data, nevertheless the relative trends are independent on the chosen defect density. In the Fig. 2 (right axis), the estimated failure probability is plotted as function as the device length. One can see that a reduction in the device length from 3 mm to 0.75 mm does lead to a 4-fold decrease of the failure rate. Therefore the reduction of the chip length has a two-fold impact on the number of chips per wafer. In Fig. 2 (left axis), the predicted maximum number of available devices per 2-inch wafer is shown. It can be seen that values as high as 10000 chips per wafers could be achieved resulting in a marginal fabrication cost per chip already on a 2-inch wafer. In addition, as mentioned in the introduction, the reduced power dissipation has a important impact on the packaging costs, which constitute a big fraction of the laser system.
Fig. 2 Right axis: failure probability as function of the laser length for a 3.5 μm-wide laser. Left axis: number of lasers available for a 2 inch wafer.
3. Optimisation of facet reflectivity
In order to reduce the laser dissipation in a distributed-feedback (DFB) laser, we decided not to increase the grating coupling or reduce the active region doping levels. Instead, we decided to focus on the optimization of the laser length and facet reflectivities to obtain low dissipation QLCs without the need for dedicated fabrication [4]. As we will see, this permitted us to fabricate devices with electrical dissipations at threshold as low as 0.5 W and still obtain lasers from the same processed wafer that could deliver up to 300mW in continuous wave operation. All the lasers were coated using metallic high reflection (HR) coatings on the back facets. In addition, partial dielectric coatings (PR) were deposited on the front facets to further reduce the threshold dissipation. The length of the devices used in this work has been selected in order to avoid an impact of the coatings on the single mode yield. The facet reflectivities have been optimized in order to provide an equivalent KL value as for 2–2.5 mm long devices. These values has been chosen since our grating is generally optimized for 2–2.5 mm long devices. No sensitive difference has been observed in the single-mode yield.
In Fig. 3, the impact of the facet’s coating is shown for a 750 μm-long and 2.5 μm-wide DFB emitting at 4.5 μm. The red curves show the characteristics of the device with HR coating of the back facet only; in blue are instead the laser performance after the dielectric front facet coating. It is interesting to see that the impact of the front coating on the laser threshold is so relevant that the laser emitted power from the front facet is not quenched due to the front PR coating as expected; actually the emitted power increases resulting in a net increase of the device’s efficiency.
Fig. 3 Light-Voltage-Current characteristics of a 750 μm-long, 2.5 μm-wide DFB laser emitting at 4.5 μm. The curves are shown before (in red) and after (in blue) the front dielectric coating. Curves of the device before back-facet HR coating are not shown since no lasing action was observed. In both cases, laser emission is single mode across the whole range.
4. Low dissipation DFBs
Different spectral regions across the mid-IR range have been chosen due to their commercial interest and low dissipation QCLs have been fabricated across the range. The various active regions used in this work are based on the two-phonon resonances or bound-to-continuum designs [6] and made by lattice matched and strain-balanced InGaAs/InAlAs on InP substrates. The wafers were grown by either low-pressure metalorganic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). All the presented laser devices are processed in buried heterostructure (BH) configuration.
In Fig. 4, all the lasers presented in this work are shown, in particular we focused on 7 different active region designs. On the right axis of the figure, the electrical dissipation is plotted at threshold, both at −30C (red markers) and at 20C (orange markers). The rollover dissipation is also shown (black markers). It can be seen that threshold powers as little as 0.3 W are observed at −30 C and at room temperature, while rollover dissipations as low as 0.7W are shown.
Fig. 4 Selected spectra of the low dissipation devices fabricated in the framework of the present work. Electrical power dissipations at threshold for -30C are shown as red markers and at room temperature as orange markers. Roll-over dissipations are shown as black markers.
4.1. First atmospheric window
In Fig. 5 (left), the power-current-voltage characteristics of a 3.5 μm-wide and 750 μm-long device emitting at 4.5 μm is shown. The device threshold current at room temperature is less than 50 mA and the rollover current is smaller than 100 mA. In spite of that, the laser lases in continuous wave operation up to 50 C and the optical power is higher than 10 mW at room temperature.
Fig. 5 Left: Light-Voltage-Current characteristics as a function of the temperature for a 750 μm-long, 3.5 μm-wide DFB laser emitting at 4.50 μm. Right: Optical power vs electrical power dissipation. In the inset, some spectra are shown at different submount temperatures.
In Fig. 5 (right), the emitted optical power is plotted as a function of the injected power. We can see that the threshold dissipation is as low as 0.35 W at −30C and 0.5 W at 20C. In the inset, due to the limited area, only one spectrum for each submount temperature is shown. Nevertheless, as it can be seen from the shape of the LI characteristic, the laser is single mode across the whole current and temperature range explored.
Similar curves are presented in Fig. 6 for a device emitting at 5.25 μm. In this case, the rollover dissipation is lower than 0.75 W resulting in a device without the need for active cooling and that can be mounted in low dissipation packages generally used for SWIR diode lasers. Optical powers of few mWs are still observed. Also in this case the spectral emission is single-mode across the whole range. It is important to mention that in this work we concentrated on the optimization of the process and the facet reflectivity. For this reason, we have decided to apply the concept also on non-optimal active region designs.
Fig. 6 Left: Light-Voltage-Current characteristics as a function of the temperature for a 750 μm-long, 6.6μm-wide DFB laser emitting at 5.26 μm. Right: Optical power vs electrical power dissipation. In the inset, some spectra are shown at different submount temperatures.
4.2. Second atmospheric window
In Fig. 7 (left), the power-current-voltage characteristics of a 10.5 μm-wide and 750 μm-long device emitting at 7.85 μm is shown. The device threshold current at room temperature is less than 100 mA and the rollover current is smaller than 240 mA. In spite of that, the laser lases in continuous wave operation up to 50 C and the optical power is higher than 50 mW at room temperature. In the Fig. 7 (right), the emitted optical power is plotted as a function of the injected power. We can see that the threshold dissipation is as low as 0.55 W at −30C and 0.75 W at 20C. In the inset, due to the limited area, only one spectrum for each submount temperature is shown. As it can be seen from the shape of the LI characteristic, the laser is single mode across the whole currents and temperatures range explored. In Fig. 8, the performances of a 1 mm-long and 12.4 μm-wide laser emitting at 8.4 μm are show. It can be seen that in this case the power dissipation is sensibly bigger than for the other devices and further improvement is still needed on the active region design. Devices at 9.2 μm are being tested as shown in the Fig. 4, but optimisation of the front dielectric coating is still ongoing and the data reported are from a device where the front facet is left uncoated while the back facet is coated with a metallic HR coating.
Fig. 7 Left: Light-Voltage-Current characteristics as a function of the temperature for a 750 μm-long, 10.5μm-wide DFB laser emitting at 7.82 μm. Right: Optical power vs electrical power dissipation. In the inset, some spectra are shown at different submount temperatures.
Fig. 8 Left: Light-Voltage-Current characteristics as function of the temperature for 1 mm-long, 12.4μm-wide DFB laser emitting at 8.40 μm. Right: Optical power vs electrical power dissipation. In the inset, some spectra are shown at different submount temperatures.
5. High output power dissipation DFBs
As mentioned above, the choice of optimising the facet reflectivities, rather than the active region doping or DFB grating coupling, allows to obtain from a single process a wide spectrum of optical powers and electrical dissipations. In order to show it, we present here two high optical power devices at 4.57 μm (Fig. 9) and 7.72 μm (Fig. 10) obtained from the same processed wafers as the low dissipation devices. The only major difference is the device length and the absence of the coatings. It is important to remind that both the devices are mounted epi-side up and that single-mode spectral emission is obtained across the whole range of operation. As it can be seen, total optical powers of more than 300 mW were obtained at both extremes of the spectral range.
Fig. 9 Left: Light-Voltage-Current characteristics as a function of the temperature for a 2.25 mm-long, 8.5 μm-wide DFB laser emitting at 4.57 μm. Right: Optical power vs electrical power dissipation. In the inset, some spectra are shown at different submount temperatures.
Fig. 10 Left: Light-Voltage-Current characteristics as a function of the temperature for a 2.25 mm-long, 10 μm-wide DFB laser emitting at 7.72 μm. Right: Optical power vs electrical power dissipation. In the inset, some spectra are shown at different submount temperatures.
6. Conclusion
In the present work, we present low dissipation quantum cascade lasers from 4.5 to 9.2 μm obtained by optimising the laser facets reflectivities. The impact of device length on laser’s failure rate is also presented showing that the light source yield increase with decreasing chip length. High power DFB lasers from the same processed wafers are also presented with optical powers as high as 300 mW for epi-side up mounted devices.
Acknowledgments
The research leading to these results has received funding from the European Union Seventh Framework Programme ( FP7/2007-2013) under grant agreement n317884, the collaborative Integrated Project MIRIFISENS.
References and links
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