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Schematic of the β-DFB laser. The laser cavity is tilted at an angle θ with respect to the cleavage planes that form the laser facets. A one dimensional distributed feedback grating is defined on the device with grating lines parallel to the facets. Lateral mode distinguishability is provided by the angled cavity, resulting in diffraction limited output perpendicular to the facet. Longitudinal mode selection is provided by the β-DFB grating, allowing for single mode operation.
Cavity and grating layout of different angled cavity lasers. (a) Angled FP: Ridge width, cavity length, and cavity tilt angle are labelled as W, L, and θ, respectively. The double-ended arrows illustrate one full roundtrip path of the light inside the cavity. The plus and minus signs define the far field quadrants. (b) α-DFB: The grating lines are parallel to the device sidewalls. (c) PCDFB: The two dimensional grating is formed by an array of dots with different periodicities in two orthogonal directions, one of which is parallel to the device sidewalls. (d) β-DFB-a: The grating lines are perpendicular to the device sidewalls. (e) β-DFB-b: The grating lines are parallel to the facets.
Scanning electron microscope images of the fabricated devices. A portion of each device is imaged at a position corresponding to the bottom right corner of the device schematics shown in Fig. 2. Surface gratings are fabricated on top of the device with a grating depth of 200 nm. The position of the QCL active region is highlighted with colour. The laser ridge is defined by etching deep channels through the laser active region. (a) α-DFB laser, (b) PCDFB laser, (c) β-DFB-a laser, and (d) β-DFB-b laser. In (d), a Si3N4 layer (dark cross section) is deposited for electrical insulation and this layer is removed on top of the device for current injection.
Spectrum and far field results of angled cavity QCLs. Testing is performed at room temperature in pulsed mode. (a) Emission spectra: Angled FP and α-DFB are multimode, spanning a wide range of wavelengths. PCDFB exhibits a broad range emission along with a narrow peak at 10.1 μm. β-DFB-a and β-DFB-b emit in single mode at 10.9 and 10.4 μm, respectively. (b) Far fields: 0° represents the facet normal direction. Positive and negative angles are defined in Fig. 2(a). All far fields are single lobed. Angled FP, α-DFB, PCDFB, and β-DFB-b emit in facet normal direction. The far field of β-DFB-a peaks at an off-normal direction of −53°.
Room temperature single mode performances of the β-DFB QCL. Testing is performed at room temperature in pulsed mode. (a) Emission spectra of three β-DFB-b devices with grating periods of 1.60, 1.63, and 166 μm. Single mode operation is obtained for all three devices with a wavelength spacing of 170 nm for adjacent grating periods. The SMSR is 30 dB. (b) Far field of the β-DFB device emitting at 10.42 μm, in comparison to the calculated diffraction limit. (c) Output power per facet and WPE as a function of the current density for three devices with straight FP, angled FP, and β-DFB cavities. The cavity length (2.3 mm) is the same for all three devices. The ridge widths for straight FP, angled FP, and β-DFB devices are 17, 200, and 200 μm, respectively. The tilt angle of both angled FP and β-DFB is 18.4°.
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