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We report an electrically pumped 1550 nm MEMS tunable VCSEL with a continuous tuning of 101 nm at 22 °C. The top MEMS-DBR with built-in stress gradient within the dielectric layers is deposited in a low-temperature PECVD chamber on an InP-based half-VCSEL, structured by surface-micromachining and electrothermally actuated for continuous wavelength tuning. With 2.6 mA threshold current, the laser shows maximum CW output power of 3.2 mW at 1560 nm. The MEMS-VCSEL operates in single-mode with SMSR > 39 dB across the entire tuning range. At 36 °C, the tuning range reaches up to 107 nm. The divergence angle of the MEMS-VCSEL is approximately 5.6° for all tuning wavelengths. The intrinsic linewidth of an unpackaged device is 21 MHz. Quasi-error-free operation at 12.5 Gbps using a directly modulated MEMS-VCSEL is reported for a record 60 nm tuning, showing the potential of the so-called colorless source in WDM applications.
© 2016 Optical Society of America
1. Introduction
Vertical-cavity surface-emitting lasers (VCSELs) with movable distributed Bragg reflector (DBR) based on microelectromechanical systems (MEMS) technology have attracted immense attention due to their single-mode operation and wide mode-hop free tuning range. In addition to VCSELs’ well-known advantages such as low power consumption, low production cost and high digital modulation rate, they have two important aspects which make them impeccable as ultra-wide tunable optical source. Firstly, in comparison to edge-emitting lasers, the shorter cavity length of an electrically pumped VCSEL enables a larger free spectral range (FSR). For a properly designed VCSEL resonator, FSR is the ultimate limit for a mode-hop free continuous tuning [1]. Secondly, due to their vertical resonator structure, it is comparatively easier to integrate MEMS and other photonic components in 2-D arrays for mass fabrication.
A quality of having wide tunability around different emission wavelengths instigates wavelength-specific new applications. MEMS-VCSELs are highly desirable optical sources in communication [2], gas sensing [3], optical coherence tomography (OCT) [4, 5], fiber Bragg-grating (FBG) sensing [6], and light ranging applications [7] due to their continuous tuning characteristics. Since the invention of MEMS-VCSELs, different actuation techniques such as electrothermal, electrostatic or piezoelectric forces have been employed to achieve wide tunability [8–12]. However, low-cost MEMS tunable 1550 nm VCSELs have not yet been available in the market, mainly due to their fabrication complexity.
As far as optical fiber communication is concerned, long wavelength VCSELs emitting around 1550 nm telecom window have accomplished a noticeable level of maturity in the last two decades. Direct modulation at 56 Gbps using a 1530 nm fixed-wavelength VCSEL has been reported [13]. On the other hand, a tunable VCSEL is a potential candidate for wavelength division multiplexed passive optical network (WDM-PON) systems with applications such as hot backup, sparing, and fixed-wavelength distributed-feedback (DFB) laser replacement for inventory reduction [14–18]. They give network designers an additional degree of flexibility to lower overall system cost concerning fiber-to-the-home and data center applications. In addition to that, the so-called colorless sources can meet the performance criteria for communication among wireless base stations and interconnects in high-performance computers and data center networks [19, 20].
In previous work, 10 Gbps back-to-back (BTB) link was demonstrated using a 47-nm-continuously-tunable MEMS-VCSEL [21]. A SiOx/SiNy-MEMS-DBR was surface-micromachined onto a short-cavity InP-based VCSEL with SiNx anti-reflection coating (ARC). However, the laser could not be tuned across the whole FSR as the ARC was partially etched due to a process related issue. With an improved fabrication technique compatible with a new AlO2 ARC, we have achieved a continuous single-mode tuning of > 100 nm which, to the best of our knowledge, is the widest tuning range for any electrically pumped directly modulated laser with ≥ 10 Gbps transmission capacity.
This paper mainly discusses the static characteristics of a tunable MEMS-VCSEL. For the first time, we have shown the tuning behavior of high-speed MEMS-VCSELs at elevated thermal operation. At 70 °C, for example, the device has a continuous tuning of 70 nm. For efficient butt-coupling into a single-mode fiber (SMF), far-field characteristics are analyzed across the entire tuning. The linewidth characteristics are measured using self-heterodyne technique and the characteristic profile is Voigt-fitted. The intrinsic linewidth is obtained to be 21 MHz which is considerably lower than previous generations MEMS-VCSELs [22]. Finally, quasi error-free operation of a BTB link is demonstrated for 60 nm tuning range at 12.5 Gbps, showing the compatibility of the MEMS-VCSEL as an emerging prospect in access networks and interconnects.
2. Device concept
The surface micromachining technology relies on a quasi-monolithic concept in which 11.5 pairs of λ0/4 (λ0 = 1550 nm) SiOx/SiNy dielectric layers are deposited onto the molecular beam epitaxy (MBE) grown half-VCSEL via inductively-coupled plasma-enhanced chemical vapor deposition (IC-PECVD). A schematic cross-section of a complete device and a SEM picture of a fully fabricated 2-D array of MEMS-VCSEL wafer is shown in Figs. 1(a) and 1(b) respectively. The buried tunnel junction (BTJ) consisting of two heavily doped p-AlGaInAs and n-GaInAs layers confines the pump current. This confirms sufficient current density through the active region. The BTJ also enables self adjusted lateral optical waveguiding. Due to the formation of this junction, the cavity in the center is longer compared to that in periphery region. Consequently, a radical index step Δneff is formed inside the BTJ. For a sufficiently high amplification of the fundamental Gaussian mode, its maximum overlap with the gain profile is required. This is achieved when the BTJ-diameter DBTJ is approximately matches the beam waist 2w0. Consequently, due to different lateral intensity distribution, higher order modes experience lower amplification resulting in higher side-mode suppression ratio (SMSR). An appropriate combination of DBTJ and MEMS curvature also assures fundamental transverse mode emission even for significantly larger aperture size compared to that of a standard fixed-wavelength VCSEL. For an essential reduction of parasitic capacitance, the semiconductor around the mesa is replaced by low dielectric-constant Benzocyclobutene (BCB) polymer. To guarantee high-temperature operation and to avoid fast thermal degradation of the modulation performance with increasing bias current, the entire device is embedded in gold pseudo-substrate. The dielectric bottom DBR and the gold heat-sink enable a reflectivity of almost 100% over the entire tuning range. The significantly shorter optical cavity length not only enhances the FSR, but also ensures high differential gain and high relaxation-resonance frequencies [23]. The interface between InP and air exhibits a reflectivity of 30% and divides the optical cavity into two coupled cavities. This additional reflectivity reduces the threshold current and, at the same time, shrinks the FSR significantly [1, 8]. The latter can be explained by simple resonator theory: FSR = λ2/(2Lo), where Lo is the optical resonator length. An ARC allows the field to penetrate more in the airgap. Thus the field intensity in the airgap becomes higher than the one in the cavity, compared to a MEMS-VCSEL without ARC. However, the longitudinal confinement factor (the fraction of field coupled with the multiple quantum well region) is also reduced. This is the reason why the threshold current increases in the ARC configuration. Thus, to maximize the tuning range, an λ0/4 thick AlO2 ARC with refractive index n ≈ 1.8 is deposited on the semiconductor–air interface, resulting in a residual reflectivity of 8% at λ0 = 1550 nm.
Fig. 1 (a) Cross-section of a MEMS-VCSEL. The conventional top mirror is replaced by an electrothermally actuatable MEMS membrane. (b) Scanning electron microscope (SEM) image of a 2-D array of a fully fabricated wafer.
The MEMS concave-bending expressed by radius of curvature (RoC) and the initial airgap Lair are mainly defined by the built-in stress gradient incorporated within the dielectric layers. The top DBR mirror exhibits a wide spectral reflectivity (≈120 nm) around the center wavelength of 1550 nm. An applied actuation current flowing through the MEMS electrode heats up the beams. This leads to the thermal expansion of the MEMS resulting in an extension of the optical cavity length. For an elaborated description of the active region design, the BTJ concept and MEMS structure the reader is referred to [21, 23].
3. Device characterizations
3.1. Tuning characteristics
The tunable spectrum of a MEMS-VCSEL is shown in Fig. 2(a). To reduce the influence of the wavelength deviation due to self-heating, the injection current Ibias is fixed at 12 mA which is higher than the highest threshold current for the entire tuning spectra. Due to the large DBTJ =14 μm, light is butt-coupled to a cone-lensed SMF. To sweep over the whole FSR, the resonance wavelength is tuned to a particular longitudinal mode’s (λm) initial value of 1514 nm corresponding to a MEMS heating current Imems = 15 mA. Now, with an increasing Imems the lasing peak is continuously red-shifted and reaches up to 1615 nm at Imems = 50 mA, resulting in a mode-hop free single-mode tuning of 101 nm with a center wavelength of 1560 nm at 22 °C heat-sink temperature. The suppressed higher order transverse mode/modes can be seen at lower wavelength values adjacent to the lasing mode. Due to the index step Δneff along the center of the BTJ, a non-tunable VCSEL inherently emits TEM00 mode for a large aperture [23]. In addition to that, the plane-concave resonator resulting from the MEMS-DBR, air-gap, semiconductor cavity and the bottom DBR generates a larger beam waist. At 1560 nm emission, the calculated beam waist 2w0 ≈ 17 μm (cf. subsection 3.3) is 3μm larger than DBTJ. However, continuous single-mode operation indicates that the cavity length and the mirror RoC coincide with the phase-fronts of TEM00 mode throughout the tuning range. Light–injection-current–voltage (L–I–V) characteristics of a MEMS-VCSEL with DBTJ = 14 μm at heat-sink temperature of 22 °C is shown in Fig. 2(b). At 1560 nm, the corresponding threshold current and the fiber coupled maximum optical power is 2.6 mA and 3.2 mW, respectively. Figure 2(c) shows the threshold current, maximum output power and SMSR for different tuning wavelengths. The SMSRs at different wavelengths are measured for a fixed bias current Ibias = 12 mA and fixed substrate temperature of 22 °C. The fiber coupled optical power is >1.3 mW for a tuning range of 97 nm (1515 to 1612 nm) while the threshold current is <12 mA. The deviation in both edges of the tuning envelope can be described as follows. Output power and threshold current both depend on the gain profile of the active medium and the reflectivity of the MEMS-DBR. The gain maximum is set during manufacturing to the center wavelength. Lower and higher wavelengths experience a lower gain [2]. A similar behavior can be observed in the reflectivity of a MEMS-DBR. A maximum reflectivity is achieved at the center wavelength due to the thickness of the single layers. Both lower and higher wavelengths experience a lower reflectivity and thus higher cavity losses. The same behavior can be seen in the fiber coupled power at different wavelengths. Because the gain decreases and losses increase the output power is reduced when moving away from the center wavelength. For the same reason the threshold current increases and eventually a higher current is necessary to achieve stimulated emission. The minimum and maximum threshold currents are 2.6 mA and 11 mA at an emission of 1560 nm and 1612 nm, respectively.
Fig. 2 (a) VCSEL spectra at a fixed bias of 12 mA under continuous-wave (CW) operation for different MEMS heating currents. (b) Light–injection current–voltage (L–I–V) characteristics of a 14 μm BTJ-diameter MEMS-VCSEL at heat-sink temperature of 22 °C. (c) Fiber coupled optical power, threshold current and SMSR for different tuning wavelengths.
The SMSR behaves similarly to the output power in terms of emission wavelength. The reason hereby is the combination of RoC and BTJ diameter. At the center wavelength the geometries are perfectly matched and the highest SMSR of 47.5 dB is achieved. For larger wavelengths the cavity length has to be increased which results in a smaller RoC and thus a lower beam waist. Higher order transverse modes now partially reach into the BTJ and receive some gain, the SMSR decreases. For smaller wavelengths the RoC is larger as the cavity length has to be decreased. The beam waist now is larger than the BTJ diameter, decreasing the differential gain of the lasing wavelength while the side-modes remain suppressed. Conclusively the SMSR decreases when moving from the center wavelength in either direction but overall still showing a highly single-mode emission with SMSR > 39 dB across the entire tuning range.
To show a statistical overview of the tuning range, 56 randomly selected MEMS-VCSELs from the same wafer are diced, glued to a silicon submount [Fig. 3(a)] and characterized. As shown in Fig. 3(b), the mean and the standard deviation of the tuning range is 86.1 nm and 11.8 nm respectively. The main factor contributing to this deviation is the usual gradient in the thickness and density of the MEMS dielectrics during PECVD deposition and results in an increasing mismatch between the center of the DBR reflectivity and the photoluminescence center along the periphery of the wafer.
Fig. 3 (a) An exemplary MEMS-VCSEL bonded on Silicon submount. (b) Tuning ranges for 56 MEMS-VCSELs from different positions of the same wafer. The devices are measured after gluing to a silicon submount.
3.2. Thermal properties
For characterizing thermal properties of the MEMS-VCSEL, the device is placed on a copper holder with an integrated Peltier element for temperature stabilization. First, the longitudinal mode shifting due to temperature increment is measured. For the fixed wavelength VCSEL with same half-VCSEL material and structure, the thermal wavelength-shift is governed mainly by the refractive index change in the resonator and to lesser extent, by the thermal expansion of the semiconductor layers. The resultant effect pushes the wavelength to higher values at a rate, dλ/dT = 0.09 nm/K [23]. However, due to the presence of the expandable top mirror, blue-shifting due to cavity shrinkage with increasing temperature is typical for a MEMS-VCSEL. As shown in Fig. 4(a), the resonance wavelength λm (where integer m is the order of a certain longitudinal mode) shows a linear decrease with increasing temperature. A linear fit reveals a wavelength shift of dλm/dT = −9.39 nm/K. That means, one can set the emission wavelength only by changing the temperature, without applying any MEMS current. A temperature change of only ≈11 °C is sufficient to tune over the whole FSR. When the temperature of the entire device (half-VCSEL and MEMS-DBR) is increased at a particular Imems, the air-gap length is decreased. The reason behind this phenomenon is the different linear coefficients of thermal expansion α for the half-VCSEL structure and the MEMS dielectrics. As mentioned in Section 2, the half-VCSEL mainly consists of Au substrate, BCB encapsulation and InP semiconductor with the coefficients αAu = 14.2 × 10−6K−1, αBCB =42×10−6K−1 and αInP = 4.6 × 10−6K−1, respectively. On the other hand, the dielectric materials SiOx/SiNy of the MEMS-DBR have much smaller coefficients αSiO =0.5×10−6K−1 and αSiN = 2.3×10−6K−1. Consequently, an increase in temperature of the whole device structure leads to a larger thermal expansion of the half-VCSEL compared to the MEMS-DBR. This causes the MEMS-DBR to be pulled toward the half-VCSEL surface and thus reducing the length of the air-gap. Thus, with an increasing temperature the peak of λm is blue-shifted until it reaches the lower limit (≈1508 nm) of the tuning envelope at 31 °C and eventually the next lower order mode λm−1 starts to lase at the upper limit of the tuning range at 1601 nm. As the RoC of the MEMS increases with increasing temperature, the neighboring transverse mode also fits to the BTJ and eventually obtains higher gain. Consequently, the SMSR of mode m−1 at 1560 nm is reduced by 6 dB in comparison to mode m+1 at the same emission wavelength.
Fig. 4 Thermal characteristics of a 14 μm aperture device. (a) Wavelength-shift over temperature for Imems = 0 mA, (b) Maximum, minimum and center wavelength of the tuning range at different temperature. (c) Tuning range as a function device temperature.
Next, the deviation in the tuning range is determined by varying the temperature from 10 °C to 70 °C at a constant Ibias = 20 mA. The spectral envelope of the tuning range is measured with an optical spectrum analyzer by holding the spectrum-maximum while tuning the wavelength by increasing Imems. The maximum and minimum values of the tuning wavelengths as well as the center of the tuning range are derived from the envelopes and plotted in Fig. 4(b). As the peak material-gain wavelength λg shifts according to dλg/dT ≈ 0.6 nm/K due to band-gap shrinkage, the initial lasing wavelength (λmin) is also influenced. The lasing in a MEMSVCSEL starts only when the gain overcomes the cavity losses. Therefore, as the gain bandwidth shifts towards higher wavelength, λmin also shifts towards higher wavelength. However, λmax is not red-shifted as it is already limited by the DBR reflectivity bandwidth. The tuning range shown in Fig. 4(c) increases from 99.5 nm at 10 °C to 107.13 nm at 36 °C. In this temperature regime the tuning range reaches almost to corresponding FSR = 108 nm at 36 °C. Hence, the FSR must have increased due to cavity shrinkage in order to increase the tuning range. The reason behind the increment of the tuning range can be explained as follows. The gain bandwidth for laser inversion has a certain range, which decreases with increasing temperature. However, the gain bandwidth is still broader than the FSR for thermal operation up to 36 °C. But with further temperature increment the gain bandwidth shrinks further as well as slightly shifts towards higher wavelength values. As a result, even though the FSR increases with higher temperature, the tuning range keeps shrinking at higher temperature regimes (above 36 °C in this case). It is worth to mention that the continuous single-mode tuning of 70 nm at 70 °C is the highest reported tuning range for any electrical pumped 1550 nm laser in this temperature regime.
3.3. Far-field measurements
With an extended cavity consisting of an air-gap and a concave top mirror, MEMS-VCSEL resonator is more complicated than a plain Fabry-Pérot resonator. The three-dimensional electric field distribution of the longitudinal modes inside the resonator can be approximated by the radial symmetric Gaussian beam equation in cylindrical coordinates:
with spot size w0 = w(z = 0) of the Gaussian beam. The Gaussian beam diverges along the propagation direction, therefore w(z) defines the beam radius r along the propagation axis z. The phase-front of the Gaussian beam transforms from a plane wave to a spherical wave for z → ∞ according to where z0 is the Rayleigh length defined by and R(z) is radius of phase-front. Thus the Gaussian mode can be defined by R(z) = RoC(z) of the MEMS-DBR and the optical cavity length Lo = ∑Lini (where Li is the geometrical resonator length and ni the refractive index of the corresponding layer, respectively. For the MEMS-VCSEL, the total optical length consists of Lo = Lsc.nsc + Lair.nair + Lmems−DBR.nmems−DBR + Lfixed−DBR.nfixed−DBR (where Lsc and Lair are the geometrical lengths of semiconductor cavity and air-gap, and Lmems−DBR and Lfixed−DBR are the penetration depths of MEMS- and bottom-DBR, respectively), so that the phase-fronts of the mode coincide with the mirror geometry [24]. This causes the plane phase-front at the fixed bottom mirror to gradually change to a spherical wave with a given spot size of Equation (4) gives a rough estimation of the beam waist of the Gaussian beam as it does not consider the multilayer VCSEL-structure [24]. Finally, the beam divergence can be calculated with a divergence angle θ0, whereThe geometrical parameters of the MEMS-VCSEL used for this experiment are shown in Table 1. The simulated penetration depths are approx. 1.5 μm and 2 μm for the MEMS- and bottom DBR, respectively. By using Eq. (4) and (5), one can calculate the spot size w0 and the divergence angle θ0. For λ = 1568 nm the calculated θ0 is ≈ 5°. The divergence angle θ0 of the VCSEL beam can be experimentally calculated from the spatial distribution of the radiated power in the far-field. The major parts of the measurement setup comprises a commercially available InGaAs p-i-n photodiode, two computer-controlled rotation stages with built-in stepper motors, and a low-noise transimpedance amplifier (TIA). The photodiode is mounted at a sufficient height (≈ 20 cm) above the MEMS-VCSEL. The TIA converts the photocurrent into a voltage which is required for further processing for a complete three-dimensional field profile. The photodiode is rotated azimuthally from −φ = −90° to +φ = +90° with a resolution Δφ =5° and from −ϑ = −30° to +ϑ = +30° polar angle with a resolution of Δϑ = 0.2°. Figure 5(a) shows the spatial mode characteristics mapped from far-field measurements for 1568 nm emission at 20 °C. At thermal rollover, the MEMS-VCSEL with the aforementioned membrane parameters still emits a circular symmetric, diffraction and refraction limited TEM00 beam. Due to rotational symmetry, the beam can efficiently be coupled into the optical fiber, which is one of the beneficial features of the widely tunable MEMS-VCSEL. Figure 5(b) shows the one-dimensional far-field pattern of the corresponding three dimensional plot in Fig. 5(a). The measurement is carried out at 20 °C for a bias current of 20 mA. The divergence angle θ0 of the Gaussian fitting curve is 5.66° at the point where the intensity falls down to 1/e2 (approx. 13.5%) of the highest peak. Despite this small divergence angle, efficient light-coupling is not possible without a lensed fiber due to the fact that the beam waist (inside the resonator) 2w0 =17μm is already larger than the standard SMF core Dcore = 8.2μm. The measured divergence angle does not perfectly match with the calculated far-field angle of 5°. This is due to the fact that Eq. (4) does not take into account the complicated coupled-cavity with multilayer structure of the MEMS-VCSEL. Also the estimation is predominantly valid for passive optical resonators, neglecting apertures, guiding elements or refractive index steps inside the cavity. Figure 5(c) shows the different divergence angles at different tuning wavelengths. It can be seen that the beam-divergence has a negligible amount of influence on the RoC of the MEMS for different tuning wavelengths (considering the measurement errors, as the setup is very sensitive to vibration etc.). It is worth to mention that in order to avoid thermal lensing effect, a low-impedance highly n-doped InP layer is used. Hence, the lateral current flow can take place around the insulating bottom dielectric DBR without substantial electrical losses and without generating heat.
Table 1. MEMS-VCSEL geometry at 1568 nm emission.
Fig. 5 (a) Three dimensional color gradient of the far-field at 1568 nm emission wavelength. (b) Relative optical intensity against divergence angle. (c) 1/e2 divergence angle for different tuning wavelengths.
3.4. Linewidth measurements
Due to short-cavity related shorter photon life time, fixed wavelength VCSELs show considerably larger linewidths compared to DFB lasers. A significant linewidth broadening arises when a movable MEMS is incorporated for wavelength-tuning. The typical linewidths of 1550 nm MEMS-VCSELs reported previously are in the range of 40 MHz to 300 MHz employing self-heterodyning technique [9, 22, 25]. Due to its susceptibility of vibrations, the MEMS mirror is disposed to unwanted vibrations which can be simulated in two ways: acoustic coupling to external vibrations and through Brownian motion. The former can significantly be reduced by a well-designed mechanical isolation. However, the latter source is inevitable; any mirror movement will frequency-modulate the VCSEL and eventually degrade the linewidth performance. Thermal noise related MEMS fluctuation results in a frequency fluctuation Δνrms of the emission wavelength according to:
where ΔνFSR/ΔλFSR are free-spectral ranges (FSR) of the VCSEL cavity, kB is Boltzmann constant, km is the spring constant of the MEMS-DBR, T0 is the temperature, c0 is the speed of light and λ0 is the emission wavelength [25]. In general, the frequency noise spectrum due to spontaneous emission is white and the resultant spectral shape is Lorentzian. If the intrinsic Lorentzian linewidth ΔνL is smaller than the average fluctuation of the Δνrms, an average linewidth , based on Gaussian distribution of the fluctuations can be defined asFor the MEMS membrane, we have measured km to be 1100 N/m which is very close to the Comsol-Multiphysics-simulated value of 1180 N/m. Considering ΔλFSR ≈ 75 nm, km ≈ 1100 N/m, T0 = 295 K and λ0 = 1550 nm one obtains Δνrms ≈ 38 MHz.
There are other factors such as radiation pressure, radiometric pressure and cavity extension in MEMS-VCSEL which are also contributing to the linewidth broadening [25]. The radiometric pressure occurs due to intrinsic absorption of the MEMS-DBR. Consequently, a small temperature difference between the two sides of the DBR builds up and the mirror tends to move toward the cooler side resulting in an increase in the cavity length. By designing a mirror with lower absorption and by vacuum-packaging the devices, one can reduce this effect. Another effect due to absorption and the subsequent heating is the length extension of the MEMS. Radiation pressure related MEMS fluctuation occurs when the DBR reflectivity is very high, which is the case for the MEMS-DBR. High cavity power can deflect the MEMS-DBR upwards resulting in the shift in the emission towards longer wavelengths.
Linewidth measurement of an on-wafer MEMS-VCSEL requires careful attention to a number of factors. Optical feedback from the MEMS mirror as well as from the fiber facet creates chaotic noise in the resonator [27, 28] and make it difficult to reproduce the experimental results. Additional Gaussian noise is added from the photodiode and the spectrum analyzer. In this experiment the linewidth of a MEMS-VCSEL is determined by using delayed self-heterodyne (DSH) interferometer technique, where the optical frequency of one arm is offset with respect to other. If the delay exceeds the coherence time of the laser, the combining beams interfere as if they originated from two independent lasers offset in frequency. The measurement is carried out on-wafer using unpackaged MEMS-VCSEL. The light is collimated using an AR coated lens and fed into a polarization-maintaining fiber (PMF) through two stages of optical isolators. The wavelength is set to 1550 nm by setting the ambient temperature to 22 °C. In this case, no heating current is allowed to flow through the MEMS membrane. The length of the decorrelation fiber is approximately 5 km and the acousto-optic modulator is set to operate at 150 MHz. The DSH beat signal (the power density spectrum) is measured by an electrical spectrum analyzer (ESA). The measurements are averaged 100 times on the ESA. With a stable commercial laser, the setup is verified.
A Voigt profile fitting, which is the convolution of the Lorentzian and the Gaussian profiles, is helpful to separate two contributions individually. Figure 6 (a) shows the measured DSH beat signal and the Voigt fitting of a MEMS-VCSEL operating at Ibias = 26 mA and Imems = 0 mA, corresponding to 1548 nm emission. The Gaussian dominated linewidth is estimated as the full-width half-maximum (FWHM) of the Voigt profile divided by which means The measured ΔνB is close to what has been obtained from Eq. (7) and is significantly less than the previously reported values for a packaged MEMS-VCSEL in [22]. This is due to the fact that the smaller dimensions of the MEMS result in a larger km. Also the half-VCSELs used in this experiments have compressively strained quantum wells active region. The effect of free carrier plasma on linewidth degradation is lower in compressively strained quantum well devices in comparison to unstrained wells [29]. The power density spectrum of the DSH beat signal for diffident Ibias is shown in Fig. 6(b). Figure 6(c) shows the Voigt fitted linewidth Δνv over the inverse optical power 1/P for 1548 nm emission, where the linewidth increases linearly with 1/P. A linear fit reveals a linewidth-power product of Δνv · ΔP = 0.62 MHz mW. Figure 6 (d) is a plot of Voigt fitted linewidths for different tuning wavelengths. The dip in 1548 nm emission accounts for the maximum optical output power at this wavelength.
Fig. 6 (a) Normalized power density spectrum of the beat note of self-heterodyne linewidth measurement of an MEMS-VCSEL with a fiber delay length of 5 km. Bias is set at 26 mA for an emission wavelength of 1548 nm. The experimental data (black dots) are fitted with a Voigt profile (red curve). The linewidth is estimated as the FWHM of the Voigt profile over . The total linewidth of this VCSEL is extracted to be 42 MHz. (b) Power density spectra of the beat signals for different operating currents. (c) Linewidth over the inverse optical power. (d) Voigt-fitted linewidth for different tuning wavelengths.
From the relationship between the Voigt spectrum with the Lorentzian and Gaussian spectra:
one can also retrieve ΔνL using so-called reference points away from the center. Since 1/f noise related broadening is endured mostly in the vicinity of the maximum, the 3-dB width strongly affected by Gaussian component is misleading. The 20-dB linewidth is characteristically dominated by the Lorentzian contribution and can be used for an educated estimation of ΔνL. Using this value as initial estimation, new 3-dB ΔνL of 21 MHz is calculated. The linewidth can further be reduced for narrow-linewidth applications like OCT by increasing the stiffness of the mirror (i.e. higher spring constant km) and by increasing both refelectivity of the MEMS-DBR as well as by increasing the air-gap (i.e. longer resonator; higher Q factor). We believe that the linewidth can be significantly reduced by using a wavelength locked packaged MEMS-VCSEL [Table 2].3.5. Tuning speed
The tuning speed of the MEMS-VCSEL plays an important role in several applications such as OCT and gas sensing. In order to monitor transient processes and to avoid fluctuating measurement conditions like beam steering (refractive index fluctuations in the measurement path) during sensing applications, scan rates of 100 Hz or higher are usually desired [3]. We have investigated tuning speed of the electrothermally actuated MEMS-VCSEL by applying a sinusoidal modulated signal which is added to a DC heating current Imems = 25 mA. Figure 7 depicts the frequency response of a MEMS-VCSEL. The laser bias current is fixed at 20 mA during the whole experiment whereas the operation temperature is set to 22 °C. The modulation depth decreases 10 dB/decade for higher frequency corresponding to a low pass filter of first order,
with the maximum tuning range of Δλ0, the modulation frequency f and the thermal time constant τ. For the device with Δλ0 = 70 nm, a fit reveals τ = 1.37 ms. At the characteristic cut-off frequency of f3dB = 200 Hz the device covers half of its initial tuning range. With increasing frequencies, the heat inside the MEMS is not fully dissipated during the modulation cycle and the tuning range converges towards zero around the central wavelength.3.6. Data transmission
Modulation analysis in the large-signal domain is essential to evaluate the performance in real digital data transmission systems. To test the feasibility of using widely tunable MEMSVCSEL as a light source we have demonstrated direct modulation of the tunable device with 75 nm tuning range. A non-return-to-zero (NRZ) signal with 12.5 Gbps pseudo-random bit sequence (PRBS) of length 215−1 is generated using an arbitrary waveform generator from Techtronix. The modulation signal with peak-to-peak voltage Vpp = 1 V is applied to the DC-biased (Ibias = 20 mA) MEMS-VCSEL. The small-signal parameters for the device under operation are summarized in Table 3. The device has a SMF coupled maximum output power of 1.5 mW at the center wavelength of 1560 nm. BER performance at a data rate of 12.5 Gbps for four diffenrt emission wavelength is shown in Fig. 8. Quasi error-free (log10BER < −9) operation has been achieved for all four wavelengths (i.e from 1530 nm to 1590 nm continuous tuning) with BTB configuration. However, the power penalty becomes substantially larger towards the edge of the tuning range (i.e. smaller than 1530 nm and higher than 1590 nm) and ends up in an erroneous transmission at these emissions. These InP-based VCSELs are still mostly thermally limited. Therefore, the modulation response is over-damped and has a smooth shape favoring higher data-rate. Moreover, 12.5 Gbps large-signal modulation performance is obtained by biasing the diode around the CW thermal roll-over. When the device is driven with 50% duty cycle pulses, the thermal rollover occurs at higher Ibias. Thus, the small-signal 3-dB bandwidth f3dB of these BCB VCSELs could be underestimated by common rules of thumb.
Fig. 8 Bit error rate performance as a function of received optical power for a NRZ direct modulation of a tunable MEMS-VCSEL. Quasi-error-free transmission at 12.5 Gbps is achieved for 60 nm continuous tuning range.
Table 2. Linewidth of a MEMS-VCSEL operating at 1548 nm, measured using direct Voigt fitting at 3-dB as well as reference points using separation method.
Table 3. Device parameters derived from small-signal analysis.
4. Conclusion
We have shown the some important static characteristics and tuning behavior of widely tunable MEMS-VCSELs at elevated temperatures. A SiNx/SiOy-based MEMS-DBR with a broadband reflectivity is integrated onto a BCB encapsulated high-speed half-VCSEL and 107.13 nm continuous tuning is achieved around 1560 nm by electrothermal actuation at 36 °C. At 1560 nm, the MEMS-VCSEL has a fiber coupled maximum output power of 3.2 mW. The device shows up to 70 nm tuning even at 70 °C. The divergence angle at 1/e2 intensity is almost constant at around 5.6° for the entire tuning range. The linewidth is determined using self-heterodyne method and an intrinsic linewidth of 21 MHz is achieved at 1550 nm. Finally, a BTB transmission using NRZ signal at 12.5 Gbps is shown for a record 60 nm tuning.
Acknowledgments
The work was funded (Grant number: 13N12519) by German Federal Ministry of Education and Research (BMBF) within the framework “KMU-innovativ: Photonik/Optische Technologien” as project VCSEL-TRX. The authors also thank Christian Gierl for his involvement in the initial development of the surface micromachining process.
References and links
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