Intra-cavity patterning for mode controlin 1.3μm coupled VCSEL arrays

Lukas Mutter, Benjamin Dwir, Andrei Caliman, Vladimir Iakovlev,Alexandru Mereuta, Alexei Sirbu, and Eli Kapon*

Ecole Polytechnique Federale de Lausanne (EPFL), Laboratory of Physics of Nanostructures,CH 1015 Lausanne, Switzerland

*eli.kapon@epfl.ch

Abstract: We report coupled VCSEL arrays, emitting at 1.3 μm wave-length, in which both the optical gain/loss and refractive index distributionswere defined on different vertical layers. The arrays were electricallypumped through a patterned tunnel junction, whereas the array pixels wererealized by intra-cavity patterning using sub-wavelength air gaps. Stableoscillations in coupled modes were evidenced for 2x2 array structures, fromthreshold current up to thermal roll-over, using spectrally resolved fieldpattern analysis.

© 2011 Optical Society of America

OCIS codes: (140.3290) Laser arrays; (140.3325) Laser coupling; (140.7260) Vertical cavitysurface emitting lasers.

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#138721 - $15.00 USD Received 29 Nov 2010; revised 6 Feb 2011; accepted 17 Feb 2011; published 28 Feb 2011(C) 2011 OSA 14 March 2011 / Vol. 19, No. 6 / OPTICS EXPRESS 4827

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1. Introduction

Vertical cavity surface emitting lasers (VCSELs) have many applications that need high-powersingle optical-mode operation. High-power VCSELs require a large aperture size, allowingseveral transverse modes to lase. Smaller aperture size leads to single mode operation, but atthe expense of reduced output power. To obtain higher single mode output power, differentapproaches of optical cavity engineering of VCSELs were employed. They include lateral pat-terning of the top distributed Bragg reflectors (DBRs) by shallow surface etching [1, 2], bymetal deposition [3,4], by etching of hole structures [5–9] or by ion indiffusion [10]. Combina-tions of oxide-defined and implanted apertures [11,12] were also employed in order to positionthe pattern closer to the active region of the cavity. More sophisticated refractive index patternshave been inserted into the optical cavities by selective etching of an intra-caity layer betweentwo overgrowth steps, realizing antiguided VCSELs [13] and coupled VCSEL arrays [14, 15].More recently, techniques for patterning the optical gain distribution using buried tunnel junc-tion (TJ) structures have been introduced [16]. This approach has yielded high performancesingle mode VCSELs, operating at telecommunication wavelengths, fabricated using doublewafer fused DBRs mirrors [17] or dielectric mirrors [18]. Combining the buried TJ approachwith a separate refractive-index patterning close to the cavity core has not been implementedyet, but would be very powerful for optimally designing the optical mode pattern and gain insuch devices.

This is precisely what we report here, using double-wafer-fused, buried TJ long wavelengthVCSELs. The gain profile is defined by patterning a buried TJ layer used for carrier injec-tion and the refractive index distribution is tailored by adding sub-wavelength air gaps intothe cavity. The combined patterning is employed for improving the mode pattern definitionand discrimination, and opens up new ways for implementing new designs of VCSEL opticalcavities.

#138721 - $15.00 USD Received 29 Nov 2010; revised 6 Feb 2011; accepted 17 Feb 2011; published 28 Feb 2011(C) 2011 OSA 14 March 2011 / Vol. 19, No. 6 / OPTICS EXPRESS 4828

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Fig. 1. Left panel: 2x2 VCSEL array structure with definition of gain (square tunnel aper-ture of side length sT J = 10 μm) and refractive index profile (pixel of side length sP = 5 μm)on separate vertical layers. Right panel: Vertical refractive index profile and electric fielddistribution inside a plain not patterned cavity.

2. Intra-cavity patterning

Figure 1 illustrates schematically the structure of the patterned-cavity VCSEL arrays.The devices were fabricated using organometallic vapor phase epitaxy of the gain In-GaAs/InAlGaAs/InP multiple quantum well (QW) region and the GaAs/AlGaAs DBR mirrors,followed by patterning and double wafer fusion [17]. Optical gain is provided by a stack of sixstrained InGaAs/InAlGaAs QWs grown on a planar (100)-InP substrate with parameters opti-mized for emission near 1320 nm wavelength [19]. The tailoring of the gain and refractive indexdistributions were implemented on the InP-based cavity core using two patterning steps. First, asquare buried TJ mesa of side length sT J = 10 μm was defined using electron beam lithographyand wet chemical etching [20], followed by regrowth of another stack of InP/GaInAsP layers.The patterned TJ mesa localizes the injected carriers and thus defines the lateral extent of thegain region. The additional stack of regrown layers provides the level at which the refractiveindex is patterned within this cavity region. Here it is provided by a set of four squares of sidelength sP = 5 μm that define four VCSEL apertures in a 2x2 array configuration, located at thecenter of the TJ mesa. Several otherwise identical structures with different array pixel separa-tions g = 100, 250, 500 and 750 nm were fabricated; a square border of width b = 3 μm wasalso introduced, defining the overall size of the array matrix (see Fig. 1). The pixel separationsand array boundary were realized using a SiO2 mask defined by electron beam lithography,through which 60 nm deep airgaps were etched using inductively coupled plasma (ICP). TheTJ pattern and the array matrix were mutually aligned (accuracy of < 100nm) by using com-mon alignment marks in the electron beam lithography steps. Since intra-cavity pattern aresupposed to have a strong impact on the cavity mode, the etched patterned was placed close toa node position of the electric field, see Fig. 1(b).

#138721 - $15.00 USD Received 29 Nov 2010; revised 6 Feb 2011; accepted 17 Feb 2011; published 28 Feb 2011(C) 2011 OSA 14 March 2011 / Vol. 19, No. 6 / OPTICS EXPRESS 4829

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3. Measurement Results and Discussion

Typical light-voltage versus current characteristics (cw operation at room temperature) of a2x2 array structure with a pixel separation of g = 250nm is depicted in Fig. 2(a). The deviceexhibits a threshold current Ith = 3.4mA and maximum output power of 1.6 mW at 17.5 mA.The maximum output power of the cavity patterned devices is still lower than comparabledevices with only TJ patterning [17,19]. This is probably due to increased scattering introducedby the refractive index pattern. Further optimization is needed to increase output power.

The near field- and far field images acquired with a CCD camera and the emission spec-tra measured with an optical spectrum analyser (HP70950A) are presented in Fig. 2(b). At aninjection current of 1 mA (below threshold) the near field shows the amplified spontaneousemission, which already reflects the defined array pixels. This pixelation becomes more pro-nounced close to lasing threshold. Due to the asymmetric electrical contacting (see Fig. 1), the

#138721 - $15.00 USD Received 29 Nov 2010; revised 6 Feb 2011; accepted 17 Feb 2011; published 28 Feb 2011(C) 2011 OSA 14 March 2011 / Vol. 19, No. 6 / OPTICS EXPRESS 4830

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VCSEL array reveals slightly asymmetric intensity distributions in the near field images belowand above threshold. Above the lasing threshold the array is coupled in the out-of-phase su-permode [4, 21, 22], for which the electric field of neighboring pixels has a phase shift of π ,yielding the four lobes in the far field intensity image [19]. This particular array lases at thesame (highest order) supermode from threshold to thermal roll-over, with the other transversemodes being at least 25 dB down in intensity. The red shift of the dominant supermode betweenthreshold and 14 mA current is due to heating of the active region (2.7 nm red shift, whichcorrespond to about 27◦C degrees increase in temperature). The devices remained coupled out-of-phase also for pumping currents exceeding thermal roll-over. Since for most applicationsin-phase emission is desirable, an additional phase shift layer could be used in order to obtainGaussian-like beam patterns as demonstrated in Ref. [23].

The emission spectra of two nominally identical devices from different wafer locations with apixel separation of g= 250 nm, measured at a pumping current of 14 mA, are shown in Fig. 3(a)and 3(b). Whereas device (a) shows a side mode suppression ratio of about 26 dB, for device(b) the ratio is less than 6 dB. This difference could stem from device variations due to nonuniformities in epitaxial layer thickness / composition as well as inadvertent pattern variations.In order to gain insight into this characteristic, the near field image was spectrally resolvedby scanning the imaged near field plane with an optical fiber connected to a high-resolution

#138721 - $15.00 USD Received 29 Nov 2010; revised 6 Feb 2011; accepted 17 Feb 2011; published 28 Feb 2011(C) 2011 OSA 14 March 2011 / Vol. 19, No. 6 / OPTICS EXPRESS 4831

(100 pm) optical spectrometer. The extracted near field mode patterns thus obtained are dis-played in Fig. 3. The intensity pattern of the main mode m1 exhibits four lobes and correspondsfor both devices well with the patterned array pixels, which are indicated by dashed lines super-imposed on the patterns. It represents the out-of-phase supermode coupling of the fundamentalmodes m′

1 of the single pixels. The lower-order supermodes built out of the m′1 pixel mode, ex-

pected to oscillate at longer wavelengths, are suppressed. This is most likely because they sufferlarger scattering losses at the etched airgaps between pixels, where their intensity is larger thanthat of the highest order supermode.

The relatively strong pixel confinement induced by the etched airgaps yields higher-ordermodes confined within each pixel, which we label here m′

l , with l = 2,3, ... Such higher or-der pixel mode, also coupled in the highest order supermode configuration, is labeled mh2 inFig. 3(a), and lases at 1308.8 nm. A similar mode, labeled md2, is observed in the spectrum ofdevice (b), near 1310.5 nm wavelength. This mode is composed this time of diagonal sub-peakconfiguration, as revealed in the corresponding spectrally-resolved near field patterns. For themd2 mode, adjacent pixels couple in mirror-symmetrical pixel configuration, yielding a pecu-liar ring field pattern. Coupled modes made of still higher order pixel modes oscillate at shorterwavelengths, near 1306.2 nm and 1308.4 nm for device (a) and (b), respectively. These modes,labeled m3 in Fig. 3, consist of coupled pixel modes showing three intensity peaks each, andextend well beyond the outer airgaps definition. For device (a) an additional lower order super-mode at 1311.8 nm appears.

The analysis of Fig. 3 sheds light on the mode selection mechanism. The modal gain is deter-mined by the overlap of the field patterns and the gain region. The modal losses are determinedlargely by scattering off the airgap structures introduced at the refractive index patterning level.The weaker discrimination against the higher-order pixel modes in device b) may result fromthe spatial profile of the gain, which better overlaps the higher order pixel mode in this particularcase. The higher order modes m3 are always well suppressed, thanks to the loose confinementof the corresponding pixel mode.

The array structures with gap width of 100 nm showed similar near and far field behavioras for the 250 nm separation, presented in Fig. 2, except that a higher number of modes wasobserved in the spectrum. This is attributed to a weaker pixel definition. On the other hand, fora pixel separation of 750 nm, the array pixels were not coupled anymore. This can be explainedby a too small penetration of the evanescent waves of the pixel modes, which yields localizationat the different pixels due to optical disorder [24].

4. Conclusions

In conclusion, the larger mode discrimination and the better conformity of the mode pattern tothe pixelation introduced by the air gaps, as compared to solely TJ patterning [19], illustratesthe advantage of the cavity patterning approach introduced here. However, further optimizationof the structure is necessary in order to increase the single mode power while keeping a largespatial mode discrimination. For this purpose, the contrast of the refractive index patterningshould be optimized to achieve the desired pixel definition while keeping the scattering losseslow. The gain distribution should also be tailored by the separate tunnel junction definition tofavor the desired mode. The same cavity patterning approach could be used for other purposes,e.g. , for controlling the spontaneous emission into the desired mode using an appropriate two-dimensional photonic crystal structure at the index patterning level.

Acknowledgments

This project was supported by the Swiss National Science Foundation (SNF).

#138721 - $15.00 USD Received 29 Nov 2010; revised 6 Feb 2011; accepted 17 Feb 2011; published 28 Feb 2011(C) 2011 OSA 14 March 2011 / Vol. 19, No. 6 / OPTICS EXPRESS 4832