Integrated high brightness array semiconductor lasers incorporating multimode interference couplers

TK Electrical engineering. Electronics Nuclear engineering

Murad, Masoud Kheder ; Bryce, Catrina

University:University of Glasgow

关键词: array, high brightness, laser, Phase locked, MMI;   

Others  :  http://theses.gla.ac.uk/4591/1/2011MasoudPhD.pdf

来源: University of Glasgow

【 摘 要 】

The research described work in this thesis is concerned with the development andrealisation of high brightness array laser diodes operating in single spatial mode. Thefabrication of the high brightness laser devices was carried out on 830 nm GaAs/AlGaAsmaterial system. Broad area lasers were fabricated to evaluate the material quality. Thematerial design was based on the high d/Г concept, by which the optical power ismaximised prior to thermal roll-over or the catastrophic optical mirror damage (COMD).A quantum well intermixing (QWI) process was developed for integrating the nonabsorbing mirrors (NAMs), the gain section, the MMI coupler and the single spatialmode output waveguide. The quantum well intermixing (QWI) was used to fabricate nonabsorbingmirrors (NAMs) with a blue shift of 58 nm. The annealing for the optimumprocess was 810º C for 90 seconds. The QWI was evaluated using the photoluminescencemethod, band gap shift of 58 nm was realised. The fabricated NAMsranged from 30 to 100 μm in length. The gain section length was set at 975 μm. In thepassive sections, the MMI and output waveguide are 1 mm long. The total device lengthwas around 2 mm. No COMD was observed in the fabricated devices meaning that thequantum well intermixing has worked well. The propagation loss measurement for 830nm passive waveguide, intermixed with a QWI blue shifted 58 nm and 9.8 mm long was4.48 dB/cm. This is comparable to the loss that was measured from broad area lasermaterial, 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 moderidge waveguide were that, a ridge depth of 1.84 μm supported a single mode. Theselected ridge width was 2.5 μm. Modelling of a 1x4 MMI array laser and a 1x2 MMIarray was undertaken using the beam propagation method (BPM). The optimum lateralspacing of the gain waveguides was found to be in a range of 2.5-3.5 μm for high poweroperation. In the 1x4 MMI array laser, the phase was modelled. The inner gain sectionshave a phase difference of π/2 with respect to the outer gain sections, while the 1x2 MMIarray laser has zero phase shift between the two gain sections. 1x4 and 1x2 MMI arraylasers were fabricated. In the case of 1x4 MMI array laser, different MMI coupler lengthswere fabricated. The MMI lengths were between 617 μm and 709 μm. The devices weretested electrically using 10 μs pulses and a 1 KHz repetition rate. They were tested to aAbstract iiicurrent level of 22xIth. The power achieved was > 440 mW in pulsed mode from thesingle 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 quantumefficiency (ηext) was 32.1 %. The MMI array laser device design with an MMI width of24 μm, length of 617 μm and gain section spacing of 3.5 μm had a strong phase lockingup to an applied current of 5.2xIth. The far-field pattern width of the central lobe of thephase locked 1x4 MMI array laser was 2.1 º measured from the array facets side. Thisvalue is comparable to the diffraction limited value of 1.96 º calculated simply from(λ/N.p). The quality factor (M2emitter) for the 2.5 μm wide single ridge emitter wasestimated to be close to 1. The beam quality factor of the 1x4 MMI array bar (M2bar) wasestimated to be 1.07. The visibility (V) of the pattern was very close to 1. The phaselocked power (P) was 152.0 mW per facet for an operating current of 5.2xIth (Ith=145mA). The corresponding brightness was 19.6 MW/cm2.sr. The operating wavelength for a1x4 MMI laser diode array was a 0.822 μm. The single emission wavelength wasmeasured from the four array side with a narrow spectral width (Δλ) of 0.22 nm at theFWHM for an operating current of 5.2xIth. The narrow spectral width of 0.22 nm for thearray was a much smaller than that for a ridge waveguide laser. The ridge waveguidelaser had a spectral width of a 0.65 nm at FWHM. This spectral width was measured for acurrent of 200 mA in pulsed mode with a 5 μs pulse width. The lasing spectra of the arrayshowed four individual peaks, when the current was increased to 6.2xIth. At this point, thearray 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 arraydevices were pulsed to a current level of 30xIth. The output power was around 332 mWfrom the single output facet. The threshold current was 85 mA. The largest optical outputpower was realised for the device with an MMI length of 480 μm. The external quantumefficiency was around 33%. The phase relationship between the gain sections for the 1x2MMI array laser is an identical one (i.e.Ф1=Ф2). The phase locking was achieved in the1x2 MMI array laser. However, the phase locking was only evident up to 3xIth (Ith=85mA) CW. The width of the central lobe of the far-field pattern was 4.49 º (equivalent to1.33x the diffraction limit). There was also a reasonable correlation between the far-fieldpattern from the measurement and the far-field pattern from the simulation. The qualityAbstract ivfactor for the emitter (M2emitter) was 1, while the beam quality factor (M2bar) of the 1x2MMI array (bar) was estimated to be 1.33. The visibility (V) of the pattern was estimatedto be around 0.5. The lasing spectra showed a single wavelength emission with a peak of823.55 nm and a very narrow spectral width of 0.3 nm at the FWHM. The optical powerat an injection current of 3.2xIth was a mere 60mW CW per facet, which corresponded toa brightness of 5.02 (MW/cm2. sr). The results for a 1x2 MMI laser array indicated thatthe length of the MMI section promoted the phase locking. An accurately designed MMIlength resulted in a narrow spectral width of 0.3 nm at FWHM for an MMI length of 480μm. The spectral width increased with a reduction of the MMI length. The spectral widthwas 0.86 nm at FWHM for an MMI length of 465 μm, whereas it increased to 3.1 nm atthe FWHM for an MMI length of 444 μm. Therefore, the phase locking and thebandwidth of the 1xN MMI array laser is a self imaging and MMI cavity lengthdependent.