Integrated high brightness array semiconductor lasers incorporating multimode interference couplers

Murad, Masoud Kheder (2011) Integrated high brightness array semiconductor lasers incorporating multimode interference couplers. PhD thesis, University of Glasgow.

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Abstract

The research described work in this thesis is concerned with the development and realisation of high brightness array laser diodes operating in single spatial mode. The fabrication of the high brightness laser devices was carried out on 830 nm GaAs/AlGaAs material system. Broad area lasers were fabricated to evaluate the material quality. The material design was based on the high d/Г concept, by which the optical power is maximised prior to thermal roll-over or the catastrophic optical mirror damage (COMD). A quantum well intermixing (QWI) process was developed for integrating the non absorbing mirrors (NAMs), the gain section, the MMI coupler and the single spatial mode output waveguide. The quantum well intermixing (QWI) was used to fabricate nonabsorbing mirrors (NAMs) with a blue shift of 58 nm. The annealing for the optimum process was 810º C for 90 seconds. The QWI was evaluated using the photoluminescence method, band gap shift of 58 nm was realised. The fabricated NAMs ranged from 30 to 100 μm in length. The gain section length was set at 975 μm. In the passive sections, the MMI and output waveguide are 1 mm long. The total device length was around 2 mm. No COMD was observed in the fabricated devices meaning that the quantum well intermixing has worked well. The propagation loss measurement for 830 nm passive waveguide, intermixed with a QWI blue shifted 58 nm and 9.8 mm long was 4.48 dB/cm. This is comparable to the loss that was measured from broad area laser material, which had a loss of 6.9 dB/cm. Fimmwave and beam propagation method (BPM) were used for the modelling. The results of the modelling for the single mode ridge waveguide were that, a ridge depth of 1.84 μm supported a single mode. The selected ridge width was 2.5 μm. Modelling of a 1x4 MMI array laser and a 1x2 MMI array was undertaken using the beam propagation method (BPM). The optimum lateral spacing of the gain waveguides was found to be in a range of 2.5-3.5 μm for high power operation. In the 1x4 MMI array laser, the phase was modelled. The inner gain sections have a phase difference of π/2 with respect to the outer gain sections, while the 1x2 MMI array laser has zero phase shift between the two gain sections. 1x4 and 1x2 MMI array lasers were fabricated. In the case of 1x4 MMI array laser, different MMI coupler lengths were fabricated. The MMI lengths were between 617 μm and 709 μm. The devices were tested electrically using 10 μs pulses and a 1 KHz repetition rate. They were tested to a Abstract iii current level of 22xIth. The power achieved was > 440 mW in pulsed mode from the single output facet. This power was equivalent to an optical intensity of 17.6 MW/cm2. The threshold current measured for the device was 145 mA. The external quantum efficiency (ηext) was 32.1 %. The MMI array laser device design with an MMI width of 24 μm, length of 617 μm and gain section spacing of 3.5 μm had a strong phase locking up to an applied current of 5.2xIth. The far-field pattern width of the central lobe of the phase locked 1x4 MMI array laser was 2.1 º measured from the array facets side. This value is comparable to the diffraction limited value of 1.96 º calculated simply from (λ/N.p). The quality factor (M2 emitter) for the 2.5 μm wide single ridge emitter was estimated to be close to 1. The beam quality factor of the 1x4 MMI array bar (M2 bar) was estimated to be 1.07. The visibility (V) of the pattern was very close to 1. The phase locked power (P) was 152.0 mW per facet for an operating current of 5.2xIth (Ith=145 mA). The corresponding brightness was 19.6 MW/cm2.sr. The operating wavelength for a 1x4 MMI laser diode array was a 0.822 μm. The single emission wavelength was measured from the four array side with a narrow spectral width (Δλ) of 0.22 nm at the FWHM for an operating current of 5.2xIth. The narrow spectral width of 0.22 nm for the array was a much smaller than that for a ridge waveguide laser. The ridge waveguide laser had a spectral width of a 0.65 nm at FWHM. This spectral width was measured for a current of 200 mA in pulsed mode with a 5 μs pulse width. The lasing spectra of the array showed four individual peaks, when the current was increased to 6.2xIth. At this point, the array is no longer phase locked. The wavelength peaks were as follows: λ1=821.35 nm, λ2=821.59 nm, λ3=821.83 nm and λ4=822.08 nm, respectively. The spectral width (Δλ) was around 0.22 nm at FWHM for the each of the individual peaks. The 1x2 MMI array devices were pulsed to a current level of 30xIth. The output power was around 332 mW from the single output facet. The threshold current was 85 mA. The largest optical output power was realised for the device with an MMI length of 480 μm. The external quantum efficiency was around 33%. The phase relationship between the gain sections for the 1x2 MMI array laser is an identical one (i.e.Ф1=Ф2). The phase locking was achieved in the 1x2 MMI array laser. However, the phase locking was only evident up to 3xIth (Ith=85 mA) CW. The width of the central lobe of the far-field pattern was 4.49 º (equivalent to 1.33x the diffraction limit). There was also a reasonable correlation between the far-field pattern from the measurement and the far-field pattern from the simulation. The quality Abstract iv factor for the emitter (M2 emitter) was 1, while the beam quality factor (M2 bar) of the 1x2 MMI array (bar) was estimated to be 1.33. The visibility (V) of the pattern was estimated to be around 0.5. The lasing spectra showed a single wavelength emission with a peak of 823.55 nm and a very narrow spectral width of 0.3 nm at the FWHM. The optical power at an injection current of 3.2xIth was a mere 60mW CW per facet, which corresponded to a brightness of 5.02 (MW/cm2. sr). The results for a 1x2 MMI laser array indicated that the length of the MMI section promoted the phase locking. An accurately designed MMI length resulted in a narrow spectral width of 0.3 nm at FWHM for an MMI length of 480 μm. The spectral width increased with a reduction of the MMI length. The spectral width was 0.86 nm at FWHM for an MMI length of 465 μm, whereas it increased to 3.1 nm at the FWHM for an MMI length of 444 μm. Therefore, the phase locking and the bandwidth of the 1xN MMI array laser is a self imaging and MMI cavity length dependent.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: array, high brightness, laser, Phase locked, MMI
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Colleges/Schools: College of Science and Engineering
Funder's Name: UNSPECIFIED
Supervisor's Name: Bryce, Prof. Catrina
Date of Award: 2011
Depositing User: Dr Masoud Kheder Murad
Unique ID: glathesis:2011-4591
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 09 Oct 2013 15:38
Last Modified: 09 Oct 2013 15:53
URI: http://theses.gla.ac.uk/id/eprint/4591

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