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.

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
• 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: https://theses.gla.ac.uk/id/eprint/4591