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We demonstrate an athermal and electrostatically-tunable 850nm-band MEMS VCSEL for the first time. The thermal wavelength drift is compensated by the thermal actuation of a cantilever-suspended mirror with a bimorph effect. At the same time, the resonant wavelength can be continuously tuned by electro-static force as a voltage is applied in the cantilever structure. A continuous wavelength tuning of 10 nm is obtained with a low thermal wavelength drift, which is 10 times smaller than that of conventional VCSELs. Our athermal and tunable VCSELs enable us to reduce the channel spacing in course wavelength division multiplexing optical interconnects even under uncooled operations.
© 2014 Optical Society of America
1. Introduction
In recent years, the demands for high capacity interconnections in data centers have been increasing in order to support the rapid growth of internet traffics [1,2]. Optical interconnections employing wavelength division multiplexing (WDM) is one of the candidates for increasing the bandwidth per fiber for future data center networking [3]. The wavelength of semiconductor lasers is strictly monitored and precisely controlled with a temperature controller in long-haul dense WDM networks so as to allocate each channel precisely, however, it is not practically allowed for short-reach systems in terms of power consumption, cost or module size. Therefore it is a key challenge to realize the athermal semiconductor lasers exhibiting a fixed wavelength even under ambient temperature changes, which will eliminate costly and bulky temperature controllers. The wavelength tunability is also needed at the same time for the precise wavelength allocation in cost effective WDM transmitter modules. There have been various reports on athermal operation of semiconductor lasers at a fixed wavelength [4–6], however, it has been difficult to realize the wide wavelength tunability and wavelength athermalization at the same time.
Micro-electromechanical system vertical cavity surface emitting lasers (MEMS VCSELs) have been attracting much interest as widely tunable light sources for short-reach optical networks [7,8], gas sensing [9] and biomedical imaging [10] since they offer wide and high-speed wavelength tuning with low power consumption. Their tuning mechanisms are classified into two categories: electrostatic actuation [11] and electro-thermal actuation [12]. In the past years many research groups have spent their effort in expanding tuning range and increasing the tuning speed of MEMS VCSELs [13–16]. On the other hand, we proposed a novel concept of vertical cavity optical filters and VCSELs with a thermally actuated micromachined mirror based on a bimorph effect for reducing its temperature dependence of resonant wavelengths [17–20]. The thermal actuation of the mirror enables us to tailor the wavelength-temperature dependence of the vertical cavity. Therefore the wavelength athermalization can be realized by designing a thermally actuated cantilever.
The advantage of our structure in comparison with other athermal lasers is that wavelength tuning and athermal operation can be realized at the same time. We reported the athermal and tunable MEMS VCSELs with continuous tuning range of 4 nm using electro-thermal actuation of the micromachined mirror while its temperature coefficient was reduced to −0.011 nm/K (7 times smaller than conventional VCSELs) [21]. However, the limited tuning range and the power consumption of electro-thermal tuning are remaining problems. While a preliminary result was reported on the manipulation of wavelength and its temperature coefficient of MEMS VCSELs with a thermally and electrostatically actuated micromachined mirror [20], the athermal and widely tunable operation has not been realized so far.
In this paper, we demonstrate an athermal and electrostatically-tunable MEMS VCSEL to realize widely tunable athermal lasers for use in WDM optical interconnects.
2. Design of athermal and tunable VCSEL with bimorph micromachined mirror
The schematic structure of the proposed MEMS VCSEL is shown in Fig. 1. The vertical cavity is composed of an 18.5 pair Al0.85Ga0.15As/Al0.2Ga0.8As distributed Bragg reflector (DBR), Al0.85Ga0.15As “strain control layer (SCL)”, a variable air gap, λ/4-thick Al2O3 anti-reflection layer, a λ-cavity including 5 GaAs/AlGaAs quantum wells, oxide confinement layer and a 40 pair bottom DBR. The anti-reflection layer that is inserted between the air gap and the half-VCSEL improves the tuning efficiency of a resonant wavelength [22]. It also works as an electric isolation layer between the MEMS component and the laser. A current is injected in to an active region through an oxide aperture between the C-shaped intra-cavity contact and the backside contact.
The unique part of our device is the thermally and electrostatically actuated cantilever which is integrated on the half VCSEL. The top mirror is supported by the cantilever which is composed of the top DBR and the SCL. Since the thermal expansion coefficient of AlGaAs material becomes smaller as increasing its Al content [23], the SCL has a smaller expansion coefficient than the average one of DBRs. Therefore the cantilever deflects downward as its temperature is increased, resulting in the blue-shift of the resonant wavelength. The wavelength shift is linearly proportional to the change in air gap thickness since the coupled cavity effect is eliminated by the AR layer [24]. In this case, the temperature coefficient of the wavelength is expressed as
where n is the effective refractive index of the lasing mode, L is the effective cavity length. A conventional VCSEL typically exhibits a temperature coefficient of + 0.07 nm/K due to the temperature dependence of refractive indices of semiconductor materials, which corresponds to the first term in Eq. (1). On the other hand, the wavelength shift due to the thermal actuation of the cantilever can be controlled by changing the design of the cantilever.The thermal actuation of the cantilever depends on structural parameters including cantilever length Lc, thickness, Young’s modulus, crystal lattice constant, and thermal expansion coefficient of each layer. We used a simplified two-layer cantilever model where the 18.5 pair DBR is replaced with a single layer having the average material parameters of the DBR. The simplified model gives us the following analytical solution of the radius R of a curvature in epitaxial layers;
where t is thickness, E is Young’s modulus, a is lattice constant, and the subscripts S and D denote SCL and DBR, respectively. This equation can be derived by applying a boundary condition for lattice matching to the standard two-layer cantilever model which is based on the Stoney’s theory [25]. The change in radius of curvature due to temperature variation is obtained by substituting the temperature dependent lattice constants a(ΔT) = a(1 + α‧ΔT) into Eq. (2). The material constants used in this calculation is obtained from [23], as shown in Table 1. The amount of thermal actuation at the free end of the cantilever ΔxTh is given byFrom Eqs. (2) and (3), we found that the thermal actuation is proportional to the square of cantilever length and the temperature variation ΔT. Therefore the athermal operation is possible if the micromachined cantilever is designed so that its thermal actuation compensates the thermal wavelength shift due to refractive changes. Figure 2(a) shows the calculated temperature dependence of wavelength as a function of a cantilever length without an applied voltage in the cantilever. The wavelength shift against cavity length change is calculated using a standard transfer matrix formula [26]. The temperature coefficient becomes smaller as increasing the cantilever length, and reaches to the complete athermal condition at the cantilever length of around 147 µm.
Table 1. Material constants and structural parameters
Fig. 2 (a) Calculated temperature dependence of wavelengths as a function of a cantilever length without an applied voltage. (b) Wavelength shift as a function of an applied voltage at 20 þC. (c) Temperature dependence of wavelengths versus wavelength shift under electrostatic wavelength tuning. (d) Wavelength tuning range with keeping a temperature coefficient 10 times smaller than that of conventional VCSELs.
Continuous wavelength tuning is performed by applying a voltage between the cantilever and the top laser electrode. An electrostatic field is formed across the air gap when a voltage is applied, which pulls the cantilever toward the substrate. As a result, the air gap becomes smaller and hence the resonant wavelength is blue-shifted. Note that our micromachined mirror is actuated thermally and electrostatically at the same time. Therefore the amount of electrostatic actuation is dependent on a temperature change. It is because the intensity of the electrostatic field becomes higher even if an identical voltage is applied when the air gap gets smaller due to the thermal actuation. As a result, the temperature dependence of a resonant wavelength is dependent on the wavelength tuning. The overall displacement caused by thermal and electrostatic actuation is obtained by solving the Eq. (4).
where b is width of the cantilever, E is averaged Young’s modulus, I is moment of inertial, ε0 is permittivity of free space, d is air gap thickness at the fixed end of the cantilever, R is the radius of curvature due to thermal actuation obtained from Eq. (2), and V is applied voltage, respectively. The similar equation was given in [27], however, the thermal actuation term x2/2R is newly added in our case. This equation is derived from a one-dimensional approximation model considering a distributed effective electrostatic force which is integrated along the length direction. The initial thermal stress distribution is neglected in the model for simplicity.Figure 2(b) shows the electrostatic wavelength tuning characteristic for different cantilever lengths at a fixed temperature. The wavelength shift is larger for longer cantilevers because the spring constant of a longer cantilever is smaller. Therefore a longer cantilever is suitable for wider wavelength tuning with a lower operation voltage. Figure 2(c) shows the relationship between the temperature coefficient and electrostatic wavelength shift for different cantilever lengths. The temperature coefficient is not constant because of the voltage dependent excess actuation that is a result of thermal actuation as mentioned above. Therefore the perfect athermal condition cannot be satisfied during the entire wavelength tuning range. Consequently there is a trade-off between wavelength tuning range and athermal operation. Here we define a Δλathermal as the wavelength tuning range where the temperature coefficient is kept less than 10 times smaller than that of conventional VCSELs, which is indicated as a hatched area shown in Fig. 2(c). The range is shown in Fig. 2(d) as a function of a cantilever length to clarify the optimum cantilever length. The tuning range reaches the maximum at a cantilever length of 137 µm which is slightly shorter than the length for perfect athermal condition shown in Fig. 2(a). It is because the temperature coefficient must be small but positive value (+0.007 nm/K) when no voltage is applied. The athermal tuning range abruptly decreases for further increase of Lc because the athermal condition cannot be satisfied for longer cantilever lengths. The athermal wavelength tuning range of 14 nm is expected for keeping for the optimum cantilever length.
The scanning electron microscope image of the fabricated device is shown in Fig. 3. The air gap is formed underneath the cantilever by using selective wet etching and critical point drying process. Our structure is suitable for low-cost mass production since it needs no extra process such as regrowth, bonding or precise optical assembling. The details of the fabrication process are described in [22].
Fig. 3 Scanning electron microscope image of a fabricated 850nm-band MEMS VCSEL with a thermally and electrostatically actuated cantilever.
3. Measurement result
3.1 Electrostatic wavelength tuning characteristic
We measured the tuning characteristic with electrostatic force actuation at a fixed temperature. The measured device has a 126 µm-long cantilever. The device was placed on a copper stage, and the temperature of the device was stabilized with a thermo-electric cooler (TEC). The injection current was fixed at 4.5 mA (4.1 × Ith at 845 nm) to eliminate the influence of deviation of self-heating on lasing wavelengths.
The emission spectra at different tuning voltages are shown in Fig. 4(a). Optical output was directly coupled into a multi-mode fiber and the spectrum was measured by an optical spectrum analyzer. Different voltages were applied between the tuning contact and the top n-electrode of the VCSEL. The wavelength was blue-shifted as increasing the applied voltage, and a continuous wavelength tuning range of over 30 nm was obtained with less than 15V. The device is below threshold at a tuning voltage of 0 V due to large gain-cavity detuning. Side-mode suppression ratio (SMSR) was greater than 30 dB from 830 nm to 841 nm. It showed multi-mode lasing because of relatively large oxide aperture (~5 µm). The SMSR was deteriorated at longer wavelengths because the difference between the net gain of the fundamental mode and that of the higher order mode was not sufficient. It can be improved by eliminating the high-order transverse modes by making the oxide aperture smaller.
Fig. 4 Electrostatic wavelength tuning characteristics. (a) Emission spectra at different applied voltages. (b) Wavelength as a function of the applied voltage.
Figure 4(b) shows the wavelength of the fundamental transverse mode as a function of the applied voltage. The experimental result is in good agreement with theory shown by a dashed line. The wavelength was well stabilized during the tuning operation, and no significant wavelength instability or hysteresis was seen.
Figure 5(a) shows light output versus current (L-I) and voltage versus current (V-I) characteristics under wavelength tuning by electrostatic actuation at three different tuning voltages. Output light was collected with lens system and the power was measured using a photodiode. Threshold currents were plotted in Fig. 5(b) as a function of wavelength. The wavelength varies when the injection current is swept due to self-heating. The wavelength indicated in Figs. 5(a) and 5(b) was measured at a constant current of 4 mA for avoiding the self-heating effect. The minimum threshold current was 1.1 mA at 845 nm. The continuous wavelength tuning range under lasing operation was 32 nm. The threshold current showed an abrupt increase at longer wavelength compared to shorter ones. It might be caused by the sharp drop of the optical gain at longer wavelengths around the band-edge. The angular deflection of the cantilever is less than 0.1° in the entire tuning range shown in Fig. 5(b). The tilt of the mirror affects the threshold but it is negligibly small in this case. In order to avoid the tilt effect, a double-supported cantilever can be used
Fig. 5 (a) L-I and V-I characteristics at different lasing wavelengths. The wavelength is measured at a bias current of 4 mA. (b) Threshold current as a function of wavelength.
3.2 Temperature dependence of wavelength
Next, we measured the emission spectra at different TEC temperatures from 20 to 50°C in order to measure the temperature dependence of the wavelength under electrostatic wavelength tuning. Figure 6(a) shows the wavelength as a function of the temperature at different applied voltages. The temperature dependence of conventional VCSELs is around +0.07 nm/K. On the other hand, our MEMS VCSEL exhibited much smaller coefficients in the entire tuning range thanks to the self-compensation of the wavelength shift associated with the thermal actuation of the micromachined cantilever. Note that the temperature coefficient is dependent on the applied voltage as discussed in the previous section. When the temperature dependence is slightly positive without applying a voltage, it became smaller as increasing the voltage and turned to a negative value. The almost perfect athermal operation was obtained at the applied voltage of 11.5-13.7V, corresponding to the wavelength tuning range of more than 10 nm. The temperature coefficient was successfully reduced to be 7 – 40 times smaller than that of conventional VCSELs. Figure 6(b) shows the emission spectra in this tuning range at different temperatures. The wavelength was almost fixed even under different temperatures, clearly showing the continuous wavelength tuning with athermal operation.
Fig. 6 Temperature dependence under electrostatic wavelength tuning. (a) Wavelength as a function of TEC temperature at different applied voltages. (b) Emission spectra at 11.5-13.7V under different TEC temperatures.
Figure 7 shows the temperature coefficient as a function of wavelength. The experimental result is in good agreement with the theory shown by the dashed line. The athermal range can be further expanded by optimizing the cantilever length as predicted in Fig. 2(d). The wavelength tuning range depends on the tuning efficiency per cantilever actuation. In order to increase the athermal wavelength tuning range, one way is to reduce the effective cavity length using a dielectric DBR mirror with less optical penetration.
While the high-speed modulation is not the subject in this paper, our device needs a larger mesa structure than conventional VCSELs, resulting in larger parasitic capacitance and hence limiting the modulation bandwidth. In order to increase the modulation speed of our MEMS VCSEL, we are able to reduce the parasitic capacitance by using proton-implantation in the mesa. Also, the photon-photon resonance effect in transverse-coupled cavities we recently demonstrated [28] can boost the modulation speed.
4. Conclusion
We proposed and demonstrated a novel tunable MEMS VCSEL which enables the continuous wavelength tuning and small thermal wavelength drift at the same time. We performed the modeling of the athermal and tunable MEMS VCSEL using a mechanical model including thermal and electrostatic actuations to clarify the design criteria. It was shown that the temperature coefficient of wavelength can be freely designed by changing the cantilever length, and hence the wavelength tuning and athermal operation can be realized at the same time by choosing the optimum cantilever length. The wavelength tuning range of 14 nm with more than 10 times smaller temperature dependence is expected.
The fabricated 850nm MEMS VCSEL exhibited a continuous wavelength tuning of more than 10 nm with keeping the temperature dependence 7 - 44 times smaller than that of conventional VCSELs, which is the first demonstration of the athermal and electrostatically tunable MEMS VCSEL. This result show a possibility of the precise wavelength control of athermal VCSELs toward TEC free low-cost transmitter modules for use in short reach WDM networks whose channel spacing can be reduced even under uncooled operations. Our approach needs no extra optical components and electrical circuits to stabilize the laser wavelength, which may meet the demands in optical interconnects for low-power consumption and small footprint.
Acknowledgment
This work was supported by JSPS KAKENHI (Grant Number S22226008).
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