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Lasers & Sources

Y-coupled quantum cascade lasers

A monolithic coupling scheme for mid-infrared semiconductor lasers demonstrates the feasibility of interferometric sensing devices and high-power laser arrays.
30 July 2008, SPIE Newsroom. DOI: 10.1117/2.1200807.1200

Spectroscopy at mid- and far-infrared wavelengths is important for environmental sensing (trace analysis, pollutants), medical diagnostics (blood analysis, breath control), and is a promising candidate for future high-security applications (airport security, identification of hazardous chemicals).1 Sensing chemical components in gases and liquids requires powerful coherent light sources with narrow spectral bandwidth and small divergence.2,3 In the past decade, this demand has promoted quantum cascade lasers (QCLs), which enhance the performance and shrink down the dimensions of today's mid- and far-infrared sensor systems.4 In contrast to conventional semiconductor lasers, QCLs can be designed to hit the target wavelength of a spectroscopic application.

The coupling of semiconductor lasers results in an increased output power,5 stable single-mode operation, and wavelength tunability.6,7 When integrated in a spectroscopic measurement system, monolithic coupling schemes can maximize the sensitivity, selectivity, and applicability of the sensor. However, a homogeneous laser beam is essential to avoid complicated optics and high losses of light. Therefore, the light inside the laser cavity has to be understood and controlled.

Figure 1 shows a sketch of a Y-coupled QCL with corresponding dimensions. Two laser waveguides (branches) spaced by 60μm merge into a single trunk towards a single facet. The Y-shaped QCLs have a waveguide width of 10μm and imply a bending radius of 8.3mm. The lasers were fabricated from a gallium arsenide/aluminum gallium arsenide heterostructure by lithography, etching, sputtering, and evaporation techniques.8 The devices were operated with 100ns current pulses at 78K. Their emission wavelength peaks at 10.5μm.

To analyze the output beams on both sides of the laser, both the single and double facets were imaged with a mid-infrared micro-bolometer camera. The image of the single facet exhibits a homogeneous circular intensity distribution. The two beams emerging from the double-facet end of the device are homogeneous and well-resolved (see Figure 1).

Figure 1. Sketch of the Y-shaped quantum cascade laser (QCL) with corresponding dimensions. Near-field images illustrate the intensity distributions emitted from (left) the single facet and (right) the double facet.

Far-field measurements of the double facet's interfering beams reflect the light distribution within the Y-shaped cavity.8,9 Figure 2 shows a two-dimensional far-field plot recorded at 13.5cm distance. In the lateral direction, φ, the plot exhibits interference fringes of a double slit experiment. Because both facets are on the lateral axis, no such fringes appear in the vertical direction. A cross-section at θ= 0° is plotted in one-dimension, which clearly shows the maxima of constructive and minima of destructive interference in the far-field intensity profile. The high contrast between the maxima and minima indicates that the emitted light at the two facets is coherent. However, vanishing far-field intensities can only be realized if both waves exhibit the exact same amplitude. The solid curve illustrates the calculated result of two interfering Gaussian beams of monochromatic light (at a wavelength of 10:5μm), which are in phase. With a lateral spacing of 60μm, the far field of those beams shows an excellent agreement with the measurement. In conclusion, the Y-junction coherently couples the two laser branches and locks the phase within the whole cavity.

Figure 2. Two-dimensional far-field plot recorded 13.5cm from the facet for a 5.6A driving current. The measurement is limited by the optical aperture of the detector. Intensity values were angle-corrected and interpolated for clarity. A cross-section at θ= 0° is plotted in one-dimensional representation (open circles) and fitted by two interfering in-phase Gaussian beams (solid line).

If the emitted light of a Y-coupled QCL is recorded at the single facet's side and compared to a Fabry-Pérot QCL of the same length, a higher output power is expected.8 Figure 3 shows how the peak optical power depends on the pulse current for both devices. Because the Y-coupled laser has a larger current cross section, it requires a higher current to reach the threshold. Yet in spite of the higher threshold, both curves reveal a similar slope efficiency of 0.10W/A, which indicates that coupling losses are small and constant with current. As a result, this coupling scheme efficiently merges the light from both branches into a single trunk. The cavity mode, which is deduced from all the above observations, is sketched in the inset of Figure 3. Here, two spatially-separated fundamental modes are laterally coupled into the same waveguide. Due to the even symmetry of the cavity mode, the output beams at the double facet are in-phase. Sketches of lateral mode profiles illustrate the near fields at the laser facets, which is in agreement with the captured images of Figure 1. Based on this cavity mode, the calculated far-field profile also matches the recorded data of Figure 2.

Figure 3. Light output versus current curves of a Y-coupled QCL (solid line) and a Fabry-Pérot (FP) QCL (dashed line) measured at 78K. The data were recorded at the single facet. A two-dimensional schematic view of the cavity mode in the Y-shaped laser is shown in the inset. The propagating waves couple. Lateral mode profiles illustrate the near fields at the laser facets. All field amplitudes are sketched and not drawn to scale.

In conclusion, we demonstrated that QCLs in a Y-shaped geometry couple coherently. The concept enhances the output power of a single-facet coherent emitter in the mid-infrared spectral range and provides a homogeneous far field, which is crucial for spectroscopic applications. The presented coupling scheme may be utilized in two applications. In one approach, more branches can be coupled into a single trunk to increase the power output of a single-facet emitter. Alternatively, two Y-junctions can be combined in a monolithic Mach-Zehnder-type cavity. Here, oscillations of the output power are observed by changing the temperature of one path of the lasing interferometer.10 The coupling scheme may be further exploited in a highly sensitive mid-infrared interferometric evanescent detector, which measures surrounding molecules due to changes of the refractive index.

The authors acknowledge the support by the Austrian FWF ADLIS, the Austrian Nanoinitiative project PLATON and the "Gesellschaft für Mikro- und Nanoelektronik" GMe.

Leonard Kristian Hoffmann
Institute for Solid State Electronics
Vienna University of Technology
Vienna, Austria
Center for Micro- and Nanostructures
Vienna University of Technology
Vienna, Austria
Gottfried Strasser
Institute for Solid State Electronics
Vienna University of Technology
Vienna, Austria
Dept. of Physics and Dept. of Electrical Engineering
SUNY at Buffalo
Buffalo, NY