Surface micromachined tunable 1.55 µm-VCSEL with 102 nm continuous single-mode tuning

: For the ﬁrst time a vertical-cavity surface-emitting laser (VCSEL) with a single-mode wavelength-tuning over 102nm in the range of 1550nm is demonstrated. The ﬁber-coupled optical output power has a maximum of 3 : 5mW and is > 2mW over the entire tuning range. The sidemode suppression ratios are > 45dB. The wavelength tuning is achieved with the micro-electro mechanical actuation of a mirror membrane fabricated with surface micro-machining for on-wafer mass production. The mirror membrane consists of low cost dielectric materials (SiO x /SiN y ) deposited with low temperature ( < 100 ◦ C) Plasma Enhanced Chemical Vapor Deposition (PECVD).


Introduction
Tunable semiconductor lasers operating in the wavelength region around 1550 nm are an attractive tool for many applications, such as fiber Bragg-grating sensors [1], gas spectroscopy [2][3][4], or telecommunications [5].Particularly, Micro-Electro-Mechanical System (MEMS) tunable Vertical-Cavity Surface-Emitting lasers (VCSELs) show high potential due to their short cavity related inherently longitudinal single-mode behavior enabling continuous broadband wavelength tunability.VCSELs have low power consumption, low threshold current and small beam divergence, which significantly simplifies the fiber coupling.Compared to edge emitters, the light emission perpendicular to the wafer surface enables on-wafer testing and production of two dimensional arrays.However, the state of the art tunable VCSELs especially at 1550 nm show tuning ranges limited to 22 nm [6], low output powers or need an external optical pump [7,8].We present a high performance electrically pumped single-mode MEMS tunable VCSEL with a record tuning range of 102 nm, realized without wafer-bonding or -gluing [9,10], enabling a cost-effective mass production.The presented technology is based on low temperature plasma enhanced chemical vapor deposition and is not restricted to 1550 nm only.Hence it is applicable for tunable VCSELs, receivers or filters [11,12] operating at different highly desirable wavelengths from 0.8 µm to 2.7 µm.For the realization of widely tunable VCSELs, the top mirror of a fixed wavelength VCSEL [13] is replaced by a MEMS movable reflector enabling a change of the optical cavity length, and hence wavelength tuning.MEMS movable reflectors are typically cantilever- [14][15][16] or membrane-structures [8] using high contrast gratings or distributed Bragg reflectors (DBRs) and are actuated by electrostatic, electrothermal or piezo-electric forces [9,10,14].Tunable VCSELs using cantilevers usually have small tuning ranges of only several nm [14], due to their tilt sensitive resonator geometry consisting of two planar mirrors.In contrast, plane-concave resonator geometries with a concave shaped top mirror exhibit a higher robustness, leading to a constant high sidemode suppression ratio (SMSR) and a nearly constant optical output power over the whole tuning range [17].These top mirrors are usually fabricated in a separate process [9,10] and afterwards combined with the "Half-VCSEL" consisting of the active semiconductor and the bottom mirror.This so called "twochip concept" requires additional process steps such as wafer-bonding or -gluing.In contrast, surface micromachined mirror membranes are fabricated directly on top of the Half-VCSEL and thus enable a cost-effective and reproducible mass production.This technology has been successfully demonstrated for VCSELs operating at 850 nm and 1550 nm wavelength.However, the achieved tuning ranges of 30 nm [18] and 22 nm [6], respectively, do not compete with VCSELs based on the two-chip concept showing a wide tuning range of 76 nm [10].In this letter we present the tuning characteristics of a surface micromachined VCSEL with wide continuous single-mode wavelength tuning.A schematic cross section of the VCSEL is shown in Fig. 1(a).The VCSEL consists of a planeconcave mirror system and an active semiconductor (Half-VCSEL).The movable top mirror, a DBR with integrated stress gradient for the concave bending, and a flat DBR at the bottom of the semiconductor form the optical resonator supporting the optical fundamental Gaussian mode.For a plane concave resonator with an optical cavity length L, and the radius of curvature R t of the concave top mirror, the beam waist w 0 at the plane mirror is defined by

Design and fabrication
with the resonance wavelength λ [18].Equation (1) does not account for the multi layer structure of the VCSEL.However, it has been shown that the beam waist derived from Eq. ( 1) can be used as a rough estimation [19].The SMSR is improved by introducing a circular buried tunnel junction (BTJ) defining a certain aperture within the semiconductor.The laser current is confined to the region defined by the circular shaped BTJ resulting in a radius dependent optical gain profile.For high amplification of the fundamental Gaussian-mode, the beam waist has to fit to the radius of the BTJ for an optimal overlap between the gain profile and the Gaussian-mode.Higher order modes show a significantly smaller overlap with the gain profile, due to their different lateral intensity distribution.This leads to a much lower amplification thus improving the SMSR [20].Besides the SMSR, the continuous single-mode tuning range is a figure of merit for tunable VCSELs.Its optimization is carried out by increasing the free spectral range (FSR), which is defined as the spectral separation between two adjacent longitudinal modes with L = qλ q /2, in which q is an integer and λ q the wavelength of the longitudinal mode of the order q: The FSR is increased by shortening the cavity length L = L air + L S + L DBR,t + L DBR,b consisting of the air-gap L air , the semiconductor L S and the penetration depth L DBR,t and L DBR,b of the optical field into the top and bottom DBR, respectively.Due to the intrinsic low gain of VCSELs, caused by the short active region, it is important that both mirrors provide a high reflectivity over the entire tuning range.Therefore we utilize dielectric DBRs with reflectivities > 99.5 % over the whole tuning range.The plane DBR at the bottom of the semiconductor cavity consists of 3.5 pairs of AlF 3 and ZnS λ 0 /4-thick dielectric layers with the center wavelength λ 0 = 1555 nm of the reflectivity bandwidth.Due to the high index of refraction contrast of ∆n ≈ 1 between the two materials, the DBR in combination with the gold substrate provides an almost wavelength independent reflectivity ≫ 99.9 % over the tuning range.The movable top DBR is a membrane structure consisting of 11.5 pairs of SiO x /SiN y [21] with a total thickness of around 5 µm.The refraction index contrast of ∆n ≈ 0.5 between SiO x and SiN y enables a reflectivity > 99.5 % over a bandwidth > 130 nm.The mirror with a diameter of 120 µm is suspended on four flexible beams (see Fig. 1(b)) with a length and width of 140 µm and 60 µm, respectively.The heating current flows through a 20 nm thick Cr/Au-metalization on the top mirror membrane and the suspension beams (other conductive materials are applicable as well).A current flowing through the metalization, which has an electrical resistance of 36 Ω, thermally expands the beams of the membrane.This increases the air-gap L air and thus the optical cavity length L proportionally to the heating power which shifts the emitted wavelength to higher values.The vertical thermal expansion of the DBR layers, which would shift the center wavelength of the reflectivity stopband, is ∼ 1000× smaller as compared to the lateral expansion of the membrane structure (ratio between the thickness of a single DBR layer and the length of the beams).For the given device, a lateral thermal expansion of 0.02 % is required to change the air-gap by 1 µm, which is sufficient to tune over the FSR.Hence the thermal expansion of the membrane has a negligible effect on the reflectivity stopband.The radius of curvature R t of the top DBR as well as the air-gap L air , is defined by the geometry of the mirror membrane and an appropriate stress gradient within the dielectric layers.These parameters have been optimized for a tunable VCSEL with a BTJ-radius of 7 µm.The radius of curvature of the mirror membrane is R t = 3.3 mm.With the initial air-gap of L air = 4.1 µm and the semiconductor cavity with an optical length of L S = 12λ 0 /4 = 4665 nm, the beam waist of the fundamental Gaussian mode is 9 µm (using Eq. ( 1)) which is 2 µm larger than the radius of the BTJ.The semiconductor/air-interface between the semiconductor cavity and the air-gap, exhibits a reflectivity of ≈ 30 % and divides the optical cavity of length L into two coupled cavities with length L S + L DBR,b and L air + L DBR,t , respectively.It has been shown that on the one hand this additional reflectivity reduces the threshold current, on the other hand it limits the tuning range significantly [19].To maximize the tuning range, the implementation of an antireflection coating (ARC) at the semiconductor/air-interface is required.Figure 2(a) compares the simulated tuning behavior of the tunable VCSEL with and without ARC (a more detailed description of the simulation method is given in [19]).Simulations show an FSR and thus a single-mode continuous tuning range of 106 nm and 70 nm, respectively.C. The red curve highlights a single emission spectrum of the VCSEL, lasing at 1505 nm with the suppressed higher order transversal modes and the next longitudinal mode at 1607 nm.The spectrum shows a FSR of 102 nm.

Fabrication
The presented tunable VCSEL consists of two basic components.The epitaxially grown semiconductor cavity (by molecular beam epitaxy -MBE) and the dielectric MEMS-DBR on top.The semiconductor cavity is made out of two n-type InP heat-and current spreading layers, each with a thickness of about 800 nm, sandwiching the compressively strained active region with AlGaInAs as basis material for the barriers and the wells.The BTJ consists of two highly doped lattice matched layers p + -AlGaInAs and n + -GaInAs.The heat is extracted out of the device by the Au-substrate which acts at the same time as the bottom contact of the VCSEL.The first process step is to structure the BTJ by dry etching.The overgrowth of the BTJ is done by metal-organic vapor-phase epitaxy (MOVPE) with InP.This technique perfectly planarizes the structure.In the following steps the p-side mesa is structured dry chemically and passivated by sputtered SiO 2 .The contact layers and the bottom mirror are evaporated (with electron beam evaporation) and covered afterwards by Au-electroplating (Au-substrate with a thickness of 50 µm to 60 µm).The electrical separation of all VCSELs is accomplished via chemical wet etching.An additional ARC made of SiON is deposited by low temperature (< 100 • C) ICP-PECVD.The surface micromachining process for the fabrication of the movable mirror membrane starts with sputtering a Ni-sacrificial layer with a thickness of 500 nm on top of the half-VCSEL.The sacrificial layer is structured by wet etching (HNO 3 solution).Afterwards, the SiO x /SiN y layers for the top mirror membrane are deposited with low temperature ICP-PECVD.The mirror membrane is completed with a Cr/Au-layer for the electrothermal tuning.The laser light is coupled out through a small opening in the Cr/Au-layer, structured with a lift-off process, in the center of the mirror membrane.The characteristic geometry of the mirror membrane is dry etched using a Ni-etchstop layer (same material as used for the sacrificial layer).After completing the dry etching process, the Ni-etchstop as well as the Ni-sacrificial layer are wet etched in a HNO 3 solution simultaneously.At the end of the process the released mirror membrane needs to be dried in a critical-point-dryer to ensure that the mirror membrane does not stick to the semiconductor surface due to capillary forces.For the characterization, the tunable VCSEL is mounted and electrically connected in a wafer prober.The light is coupled into a multi-mode fiber (with negligible coupling losses) which is connected either to an optical spectrum analyzer or to an optical power meter.Figure 2(b) shows the emission spectra of the tunable VCSEL for different heating powers.The red curve highlights a single emission spectrum with the lasing mode at 1505 nm.Right next to the lasing mode one can identify the suppressed higher order transverse modes.The suppressed peak at 1607 nm is the next longitudinal mode with its transverse modes.The VCSEL starts to lase at 1584 nm and is tuned to 1606 nm with a heating power of 12 mW.At this point the next longitudinal mode starts to lase at 1504 nm.For 12 mW to 63 mW heating power, the wavelength is continuously tuned over 102 nm which is the largest single-mode tuning range reported in literature for tunable VCSELs.The measurements are in good agreement with the simulations as can be seen in Fig. 2(a).The tuning range is limited only by the FSR of 102 nm.The FSR can be increased by further reducing the initial air-gap length.The implemented BTJ and the optimal adapted geometry of the top mirror provide an SMSR > 45 dB over the entire tuning range.The SMSR at the center wavelength of 1550 nm as well as the tuning range as a function of the VCSEL current are shown in Fig. 3.It is noteworthy that at a laser current of only 5 mA the tuning range still covers 82 nm with a SMSR > 40 dB and an output power > 0.4 mW.The output power and voltage as a function of the laser current is shown in Fig. 4(a).At a laser current of 27 mA the maximum output power is 3.5 mW at 1550 nm.The measured threshold currents and the maximum output powers at different wavelengths are shown in Fig. 4(b).The tunable VC-SEL shows a minimum threshold current of 2.3 mA at 1550 nm, and an output power > 2 mW over the entire tuning range.The electrothermal frequency response of the tunable VCSEL has been investigated for a sinusoidal modulated heating current, at a laser current of 25 mA.The generated heat cannot dissipate fast enough for high modulation frequencies resulting in a low pass behavior.At the characteristic cut-off frequency of f 3dB = 215 Hz, the VCSEL covers a tuning range of about 45 nm as shown in Fig. 5.The modulation depth decreases with 10 dB/decade for higher frequencies.This corresponds to a first order lowpass filter given by

Characterization
with the maximum tuning range ∆λ 0 , the modulation frequency f and the thermal time constant τ.A fit of Eq. (3) to the measurement data is shown in Fig. 5 with ∆λ 0 = 87 nm and τ = 1.3 ms.

Conclusion
We demonstrated a micro-electro mechanically tunable VCSEL consisting of an InP half-VCSEL and a surface micromachined mirror membrane with a single-mode tuning range of 102 nm.This value has been achieved by reducing the cavity length significantly compared to given concepts and by implementing an antireflection coating.The tunable VCSEL has a peak output power of 3.5 mW and a SMSR > 45 dB over the entire tuning range.A smaller beam waist (current beam waist of 9 µm) fitting to the radius of the BTJ (7 µm) of the VCSEL could further improve the threshold current and output power (larger overlap between optical field and gain profile).The presented surface micromachining technology is not limited to VCSELs emitting at 1550 nm and can be adapted to other VCSELs emitting in the wavelength regime from 0.8 µm to 2.7 µm.

Fig. 1 .
Fig. 1. a, Cross section view of the surface micromachined tunable VCSEL.b, Scanning electron microscopy of tunable VCSELs fabricated on wafer.Each VCSEL has a footprint of 480 × 480 µm 2 .

Fig. 2 .
Fig. 2. (a) Measured wavelength in comparison to simulations of the VCSEL for different heating powers and the corresponding air-gap lengths.The emitted wavelength is continuously tuned over 102 nm.(b) Emission spectra for different tuning currents and the tuning range as the envelope of the fundamental laser peak.The laser current is 25 mA and the VCSEL is stabilized at 20 • C. The red curve highlights a single emission spectrum of the VCSEL, lasing at 1505 nm with the suppressed higher order transversal modes and the next longitudinal mode at 1607 nm.The spectrum shows a FSR of 102 nm.

Fig. 3 .
Fig. 3. Tuning range and SMSR at a wavelength of 1550 nm for different laser currents.

Fig. 4 .
Fig. 4. (a) The blue curve shows the fiber coupled optical power and the red curve the voltage at the VCSEL.Both as a function of the VCSEL current measured at a wavelength of 1550 nm.(b) Threshold current and output power at the thermal roll-over for different wavelengths.

Fig. 5 .
Fig.5.Electrothermal frequency response of the tunable VCSEL.A sinusoidal a.c.modulated heating current flows through the top mirror (peak to peak current 20 mA and an offset current of 30 mA).The measured frequency response of the corresponding tuning range is shown by the red squares.The blue solid line is the fit of the first order lowpass given by Eq. (3) to the measurement.