Large-area broad band saturable Bragg reflectors using oxidized AlAs in the circular and inverted mesa geometries in oxidation of AlxGa1-xAs-GaAs depending on its from quantum well on a buried patterned AlAs layer

A semiconductor Saturable Bragg Reﬂector (SBR) is a mirror structure comprising alternating layers of high and low refractive index materials with an incorporated saturable absorber. SBRs can be used to initiate and sustain ultra-short pulses in various laser systems. In order to form ultra-short pulses, SBRs with high reﬂectivity over a broad wavelength range are required. Furthermore, large-area SBRs facilitate easy integration in a laser cavity. One of the key elements for the realization of broad band SBRs is the development of the thermal oxidation process that creates buried low-index Al x O y layers over large areas. The design, fabrication, characterization, and implementation of broad band, high index contrast III-V/Al x O y SBRs in the form of circular mesas, as well as inverted mesa structures, is presented. V C 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.


I. INTRODUCTION
The thermal oxidation of Al x Ga 1Àx As with high Al content (x > 0.85) 1 is an enabling technology in the fabrication of optoelectronic, electronic, and photonic devices such as edge-emitting lasers, 2 vertical cavity surface emitting lasers (VCSELs), 3 metal semiconductor field effect transistors (MESFETs) that are based on GaAs, 4 as well as III/V-oxide broad band Saturable Bragg Reflectors (SBRs). The low refractive index of Al x O y , the stable oxide that is created by thermal oxidation, makes Al x O y extremely useful in applications where a high index contrast is required. In the majority of the work that has been reported to date, the oxidation of Al-based materials occurred in relatively small areas. In the oxidation of VCSELs for the formation of oxide apertures, 1-4 the oxide extent does not have to exceed a few tens of microns in order to satisfy the purposes of current and optical confinement. In contrast, the wet thermal oxidation of the large-area SBRs extends over hundreds of microns.
SBRs can be used as the primary mode-locking element, or as the starting mechanism for soliton mode-locked fiber lasers and solid state Kerr-Lens mode-locked (KLM) systems. 6 With an absorber to initiate pulsing, the critical cavity alignment that is otherwise required for Kerr lens modelocking is relaxed. 7,8 Typically, in standard SBRs, the absorbers are integrated onto AlGaAs/GaAs Bragg mirrors, and the low-index contrast (Dn $ 0.5-0.6) between the layers results in narrow bandwidth mirrors ($70 nm, R > 99.5%). This paper focuses on the development of large-area SBRs, which use high index contrast (Dn $ 1.8) Bragg mirrors to achieve high reflectivity over a broad wavelength range for the realization of a laser that is widely tunable and that is capable of femtosecond pulses. Fig. 1 summarizes the tuning results that have been obtained using a broad band oxidized SBR, which contained 7 pairs of Al 0.19 Ga 0.81 As/Al x O y as the Bragg stack and a 6 nm thick strained In 0.14 Ga 0.86 As quantum well (sandwiched between Al 0.19 Ga 0.81 As barriers) as the absorber section to mode-lock a Cr:LiSAF laser. Continuous tuning over 105 nm was achieved, the broadest tuning range that has been reported from any SBR modelocked femtosecond solid state laser.
At the core of broad band SBRs is the stable conversion of AlAs to lower index Al x O y via lateral wet oxidation. [9][10][11][12][13][14][15][16][17] In order to ensure process controllability, a physical model for the oxidation of SBRs has been developed. The rate of oxidation is found to be dependent not only on the oxidation temperature but also on oxidation time, mesa geometry, and AlAs layer thickness. Using the model, the first Inverted Mesa SBR (IMSBR) that is centered at 1550 nm has been designed, fabricated, characterized, and implemented in a laser cavity. IMSBRs have the advantage of easier beam alignment as compared to the previously demonstrated 500 lm-diameter circular oxidized SBRs. 9 As illustrated in Fig. 2, 99% of the surface area on an inverted structure is usable as a SBR.

A. Design
A semiconductor-based SBR consists of a saturable absorber that is integrated on a Distributed Bragg Reflector (DBR), which is formed by growing multiple layers of alternating indices of refraction such that each layer boundary causes a partial reflection of the incident optical wave. The saturable absorber initiates and sustains mode-locking by introducing an intensity-dependent loss in the laser cavity. Each SBR is uniquely designed to meet the mode-locking requirements of the laser in which it is to be implemented. A well-designed SBR must meet the following criteria: • The absorber can be integrated onto the DBR. • The absorber material of choice features saturable absorption at the laser wavelength. • The device structure and materials selection enables the choice of key parameters such as recovery times and saturation fluence. • The SBR has low insertion loss.
• The SBR has a large enough incident area so as to mitigate the heating effects of two-photon absorption. Thermally conductive packaging can also be used to conduct heat away during operation.

B. Fabrication
Large-area, wet oxidation of Al x Ga 1Àx As SBR structures is a relatively straight-forward process that involves exposing high Al-content layers to water vapor in an inert environment at elevated temperatures. The oxidation system that was used incorporates a water vapor source and a 2-in.diameter single zone quartz tube furnace. The water vapor is supplied by bubbling N 2 through a flask of de-ionized water, which is immersed in a water bath that is maintained at 85 C. The water vapor is carried by N 2 gas from the bubbler into the heated furnace. The oxidation temperature was varied between 420 C and 450 C.
The as-grown oxidizable SBR structures consist of a seven-pair GaAs/AlAs Distributed Bragg Reflector and a 60 nm thick In 0.53 Ga 0.47 As absorber that is clad by GaAs. The structure was grown by Molecular Beam Epitaxy (MBE) on GaAs (100) substrates. Mesas were patterned immediately before oxidation by using photolithography followed by a wet etch that is comprised H 2 SO 4 :H 2 O 2 :H 2 O in a 1:8:40 ratio, respectively. Oxidation begins at the exposed mesa edge and proceeds inwards by 250 lm for full oxidation of the 500 lm-diameter mesas and $80lm in the inverted structures as shown in Fig. 2. At elevated temperatures, aluminum atoms readily react with water molecules to form Al x O y . The oxidation process is not reaction-rate limited, but rather, is limited by the ability with which the oxidant can diffuse across the already-formed oxide to the buried AlAs [stage 2]. fully oxidized circular SBR mesa and an inverted mesa structure, respectively, that were designed to mode-lock an Erbium-doped fiber laser. In 0.53 Ga 0.47 As served as the saturable absorber layer for 1550 nm. The SEM image in Fig.  3(a) indicates the conversion of AlAs to Al x O y by the distinct polycrystalline/single crystal nature of the oxidized SBR layered structure.
The motivation for developing Al(Ga)As/Al x O y SBRs is the increase in the high reflectivity bandwidth. In Fig. 4, the bandwidth of the GaAs/Al x O y SBR is much broader (approximately four times) than in the Al(Ga)As/GaAs SBR. The reflectivity spectra in Fig. 4 also reveal a slight linear absorption loss that originates from the absorber layer in both the broad band and the narrow band AlGaAs/GaAs SBRs.
The reflectivities from the circular SBRs, as well as IMSBR devices have been measured. Fig. 5 shows that the measured reflectivities from the circular SBR and IMSBR devices agree very well with the theoretical reflectivity, as well as with each other, confirming that the reflectivity of the fully fabricated devices is independent of the device geometry since the layers are identical. All three curves reveal broad reflectivity bandwidths of $600 nm with saturable losses originating from the absorber layer.
The circular SBRs were incorporated into a free-space section of a soliton mode-locked, free space lens coupled Er-doped fiber laser. Steady mode-locking was initiated using the circular SBRs at a threshold 980 nm wavelength pump power of 270 mW, and was maintained as the pump power was increased to $415 mW. Fig. 6 is an optical spectral analyzer (OSA) trace of the laser's output spectrum at a pump power of 400 mW, showing a 3-dB spectral bandwidth of 8.4 nm, which corresponds to a transform-limited pulse duration of 296 fs. The laser's fundamental repetition rate was 314 MHz [Fig. 6, inset], which is determined by the length of the laser cavity. A wide span RF trace [not pictured] showed equal amplitude of the fundamental frequency and its harmonics, thus, demonstrating that the laser was in single pulse mode-locking operation. In this fiber laser, the full broad bandwidth of the SBR could not be fully demonstrated as the spectral bandwidth is mostly determined by the balance of the dispersion and self-phase modulation. Additionally, continuous broad band mode-locking tunability is possible by the use of oxidized SBRs and would be limited only by the location of the absorber band edge and the gain bandwidth of the laser.
Similar mode-locking results using IMSBRs have been demonstrated at a pump power of 275 mW and were maintained as the pump power was increased to $490 mW. Fig. 7 is an OSA trace of the laser's output spectrum at a pump power of 490 mW, showing a 3-dB spectral bandwidth of 7.5 nm, which corresponds to a transform-limited pulse duration of 336 fs. The laser's fundamental repetition rate was 312 MHz [Fig. 7, inset]. The easier alignment of the laser to virtually anywhere on the SBR, rather than specific mesa sections, makes IMSBRs more practical for integration in real-world ultrafast lasers.
As part of the investigation of possible limitations to the implementation of oxidized SBRs, the non-saturable losses of the SBR were measured. Non-saturable losses, which are typically undesirable and do not contribute to mode-locking, are caused by absorption of the optical signal by the Bragg mirror, or by scattering from rough surfaces. The nonsaturable losses were quantified using a three-part experiment whereby the reflectivity of pulses that were emitted from a Ti:sapphire-pumped optical parametric oscillator were measured as a function of fluence. The pulse parameters were: pulsewidth ¼ 180 fs, k o ¼ 1550 nm, and repetition rate ¼ 80 MHz. The oxidized SBRs were moved in and out of the focal plane of the incident optical signal by a small distance, and a reflectivity trace was taken in each position. The results that are shown in Fig. 8 show data for traces at 9.5, 10, 10.5, and 11 mm from the focussing lens that precedes the SBR.
For fluences above saturation, a maximum reflectance of $98% relative to an AlGaAs/GaAs dielectric mirror was measured for the SBR mesa at 9.5 mm from the lens. This result estimates a non-saturable loss of $2%. The small nonsaturable loss of the fabricated broad band oxidized SBRs makes them a good candidate for ultrafast pulse generation.

IV. CONCLUSIONS
IMSBRs, which enable the generation of widely tunable ultra-short pulses, have been developed. In addition to possessing broad reflectivity bandwidths, large-area IMSBRs facilitate easier beam alignment as compared to circular SBRs, making them especially attractive for the implementation in real-world ultrafast lasers. Optical characterization shows similar mode-locking performance in the circular mesa and inverted geometries. Currently, the inverted mesa approach is being extended to the development of IMSBRs at various wavelengths, where other broad band laser materials are available.
FIG. 8. Reflectance traces from an oxidized circular mesa as the fluence is increased at four positions. A non-saturable loss of $2:5% is recorded 9.5 mm away from the focussing lens.