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Several Applications for tunable laser diodes have strict constraints in terms of overall power consumption. Furthermore, the implementation in harsh environments with large temperature fluctuations is necessary. Due to the constraint in power consumption, the application of active cooling might not be an option. For this reason we investigate the temperature characteristics of an electrically pumped MEMS-VCSEL with wide continuous wavelength tuning. For the first time, a mode hop free single mode (side mode suppression ratio (SMSR) > 40dB) tuning range of 45nm at 70°C is demonstrated with a MEMS-VCSEL. An increase of the tuning range from 85nm at 20°C to 92nm at 40°C is measured and explained. In contrast to fixed wavelength VCSEL, the investigated device shows a negative temperature induced wavelength shift of −4.5nmK−1, which is caused by the MEMS-mirror. At 1560nm, the fibre-coupled optical output power is above 0.6mW over the entire temperature range between 20°C to 70°C and shows a maximum of > 3mW at 20°C.
© 2014 Optical Society of America
1. Introduction
Wavelength tunable semiconductor laser diodes, in particular vertical-cavity surface-emitting lasers (VCSEL) [1] are advantageous for a variety of applications. In fibre Bragg-grating sensors, a single tunable VCSEL can address numerous gratings with spectrally separated reflectivities [2]. The high spectral purity of laser emission makes them predominant compared to broadband light sources and their tunability beats edge emitting diodes. In gas sensing systems, tunable VCSEL are deployed due to their large tunability, which enables the detection of several absorption lines or even different species [3–5]. And in optical coherence tomography, optically pumped broadband tunable VCSEL are deployed to increase the axial resolution and imaging depth [6, 7]. However, in telecommunication networks such as wavelength division multiplexing passive optical networks (WDM-PON), tunable VCSEL still need to be improved in terms of tuning range and temperature stability. The tuning range of VCSEL operating at the telecom wavelengths around 1550nm has reached values of 15nm by tuning the pump current from above threshold to thermal roll-over [8]. The disadvantage of this scheme is that a large variation in tuning current naturally leads to a significant change of the output power. Alternatively, the wavelength can be tuned by changing the device temperature. In this way the change of the output power is not as significant as for current tuning, but the device needs an active temperature control unit to adjust and stabilize the device temperature. The speed of thermal tuning is orders of magnitude slower compared to current tuning and the additional electrical power needed for temperature control might not comply with telecom specifications such as SFP+ allowing 1.5W (Power Level 2) or 1.0W (Power Level 1) for the complete SFP+ transceiver [9]. Therefore, a device which does not necessarily need active cooling (at least over a fraction of the temperature range to reduce the maximum needed cooling power) and is operable over a large temperature range is highly desirable. Wavelength stabilization will be either integrated into the TOSA package of the VCSEL or managed by the Optical Line Terminal (OLT). The integrated solution is based on a second monitor photo diode (the first photo diode measures optical power variations) combined with an Etalon, which can be integrated into the TOSA package. The transmission function of the Etalon is wavelength dependent and can be used as error signal for wavelength stabilization. In the latter approach, the wavelength in measured at the OLT and the error signal is sent back to the network user over a pilot tone (method will be standardized in ITU-T Q.6/15 G.metro and in ITU-T Q.2/15 and FSAN G.989.2).
Besides the mentioned disadvantages of common edge emitting diodes and VCSEL, the realizable tuning range does not cover the specified L- and/or C-band which cover a wavelength regime from 1525nm to 1565nm and 1570nm to 1610nm respectively. In consequence of the limited tuning as well as the problems with current and temperature tuning, a micro electro-mechanical wavelength tuning has been developed [10, 11]. One of the VCSEL mirrors is replaced by a micro electro-mechanical system (MEMS), which basically is a movable mirror. With this special mirror, the length of the optical resonator is changed, which shifts the resonance and thus the emitted wavelength of the MEMS-VCSEL. Since the invention of MEMS-VCSEL, different MEMS-technologies have been developed and are still being improved [12, 13]. Mirrors in form of movable cantilever structures which are actuated by means of piezoelectric [14], electro thermal [15] or electro-static forces [16]. The latter concept uses a high contrast grating (HCG) as a polarization dependant mirror. Besides other conceptional drawbacks, the disadvantage of cantilever structures is an increasing tilt of the mirror structure during tuning, restricting the tuning range (and side mode suppression ratio) to values which are smaller than those achieved for current or temperature tuning of standard VCSEL. A much better stability is provided by mirror-membranes mounted on two or more beams. The tuning range of MEMS-VCSEL based on HCG was improved to 15nm [17] which is still not larger as standard VCSEL tuned with laser current or device temperature. Other concepts use distributed Bragg mirrors (DBR) based on semiconductive [18] or dielectric materials [19]. Besides the fabrication concept of surface micromachining, which has been used for the mentioned MEMS-VCSEL, bulk micromachined MEMS-VCSEL have been demonstrated as well [20,21]. However, these technologies are still limited in wavelength tuning and they are not cost effective. As a matter of principle, the largest tuning ranges have been achieved with optical pumped VCSEL so far [6]. Since no electrical current needs to be applied, no current induced device heating and current crowding need to be taken into account for the device concept. This enables the fabrication of very short cavities having a large free spectral range, which is the ultimate limit for mode-hop free tuning. Thus, these devices are limited by gain bandwidth only. Furthermore, optical pumping can not be accepted for WDM-PON standard due to power consumption (of pump source) and complexity.
In [22] we demonstrated the current world record in wavelength tuning of 102nm for electrically pumped VCSEL. Besides a large tuning range, the influence of temperature on the device performance is a crucial point. A device without active temperature control is highly desirable to reduce the overall power consumption. Therefore, the influence of temperature on the device performance of electrically pumped MEMS-VCSEL, is investigated in this paper for the first time.
2. Device concept
A cross section view of the presented long-wavelength MEMS-VCSEL in shown is Fig. 1. The MEMS-VCSEL combines an InP based half-VCSEL with a movable MEMS-DBR. Both parts incorporate an air gap of alterable length Lair, with an initial length of around 4μm. The MEMS-DBR is processed via surface micromachining on top of the half-VCSEL using a Ni sacrificial layer. The DBR is based on 11.5 pairs of SiO/SiN, which are deposited on top of the sacrificial layer with PECVD. The MEMS-DBR shows a reflectivity bandwidth of 145nm for a reflectivity above 99.5%. A Cr/Au based heating electrode is thermally evaporated on top of the DBR for the electro thermal actuation of the MEMS. The laser light is coupled out through a circular opening in the Cr/Au electrode, which is implemented via liftoff process. A dry etching process gives the MEMS-DBR the appropriate shape of a circular dish with a diameter of 120μm suspended on four flexible beams with a length and width of 140μm and 60μm respectively. The MEMS-DBR is released by wet chemical etching of the sacrificial layer followed by critical point drying. A concave bending of the released MEMS-DBR is achieved by an intrinsic stress gradient in the multi layer structure. The stress gradient is adjusted by means of the deposition parameters of the PECVD. The concept and advantages of plane-concave VCSEL-resonators are described in more detail in [22]. A microscopic top view on a fully processed wafer of MEMS-VCSEL, which illustrates the characteristic shape of the movable MEMS-DBR, is shown in Fig. 2. Additionally, it shows the measurement setup consisting of electrical contact needles for the application of the VCSEL (n-contact on top / p-contact on substrate side) and MEMS-tuning current as well as the optical multi mode fibre for coupling the VCSEL light.
Fig. 1: Schematic cross section of the MEMS-VCSEL, which consists of a movable MEMS-DBR and an InP-based long-wavelength half-VCSEL.
Fig. 2: Microscopic view on top of a fully processed wafer of MEMS-VCSEL. A single MEMS-VCSEL is electro-optically connected for the device characterization.
The wavelength is tuned by electro-thermal heating of the MEMS-DBR. The thermal expansion lifts the mirror and extends the overall cavity length, which shifts the optical resonance. The wavelength λ is shifted to higher values for an increasing heating current ΔIMEMS. The wavelength shift Δλ is proportional to the applied heating power Pheat with . The variable R represents the electrical resistance of the Cr/Au-electrode on top of the MEMS-DBR. Detailed measurements of the tuning characteristics, such as the thermal limitation of the tuning speed to approx. 200Hz, are explained in [22]. However, fast wavelength tuning is not an essential criteria for telecom application, since the wavelength is tuned to the appropriate channel wavelength and stabilized. If the channel address has to be changed, the wavelength can be switched to the new channel within several ms.
The active region of the half-VCSEL consists of 7 compressively strained AlGaInAs quantum-wells (QWs) separated by lattice-matched AlGaInAs barriers. The application of a buried tunnel junction (BTJ), which converts electrons into holes, enables the substitution of p-doped by n-doped materials [23]. This reduces the serial resistance and thus the overall thermal heating of the device as well as free carrier absorption. The highly doped BTJ is placed at a minimum of the optical field to minimize optical losses. The QWs and the BTJ are embedded between two n-type InP layers acting as heat- and current spreaders with excellent thermal conductivity of λth = 68WmK−1. Typically, the DBR of InP based VCSEL consist of epitaxial InGaAlAs/InAlAs with a low thermal conductivity of ≈ 4.5WmK−1. In combination with a small refractive index contrast Δn < 0.3, more than 40 layers are needed to achieve a reflectivity beyond 99.5% leading to a thick stack of layers with a bad thermal conduction. To reduce the number of pairs, a fluoride and sulfide based DBR with a greatly improved Δn > 0.95 is implemented into the presented MEMS-VCSEL. In combination with the electro-plated gold substrate (p-contact), 3.5 pairs (or 7 layers) are sufficient to reach a reflectivity of ≈ 100%. Although the thermal conductivity of the fluoride and sulfide materials (2WmK−1 to 7WmK−1) is similar to the ternary InGa(Al)As/InAlAs layers, the considerable reduction in the overall thickness of the dielectric DBR reduces the thermal resistance compared to semiconductive DBR significantly. The second advantage is the large reflectivity bandwidth of > 350nm for a reflectivity R > 99.8%, which is important in terms of large wavelength tuning. In comparison, a common as-grown 65 layer InGa(Al)As/InAlAs DBR has a bandwidth of ≈ 50nm (R > 99.8%) only, greatly reducing the capability of the MEMS-VCSEL for large wavelength tuning.
3. Thermal properties of MEMS-VCSEL
The thermal properties of the MEMS-VCSEL are tested on-wafer. The VCSEL is connected with electric-contact needles and the light is coupled into an optical fibre, which is either connected to an optical spectrum analyzer (OSA) or a power meter. The wafer temperature is actively stabilized with a temperature controller unit and can be adjusted in the range from 20°C to 70°C. In the first measurement the tuning range of the MEMS-VCSEL at different temperatures was determined at a constant laser current of 30mA. The emission spectrum was measured with the OSA. Holding the maximum of the measured spectrum while tuning the wavelength by increasing the heating current through the MEMS-DBR, gives the spectral envelope of the tuning range. Figure 3 shows the tuning ranges (envelopes) for the device temperatures 20°C, 40°C, 60°C and 70°C. The tuning ranges of 92nm at 40°C, 80nm at 60°C and 56nm at 70°C present the current state of the art high temperature performance for electrically pumped MEMS-VCSEL.
Fig. 3: Envelopes showing the tuning range for different device temperatures. The VCSEL current is set constant to 30mA. An exemplary emission spectrum (at 20°C) of the MEMS-VCSEL emitting at λq = 1565nm (and its adjacent higher order longitudinal mode λq+1 = 1480nm) is shown as well.
Figure 3 also shows an exemplary single emission spectrum of the MEMS-VCSEL at λq = 1565nm, at which q is an integer and gives the order of the longitudinal mode. The laser emission is single-mode, since the transverse side modes (left side of the main emission peak) and the higher order (q + 1) longitudinal mode λq+1 ≈ 1480nm with its side modes are suppressed by > 40dB. The spectral distance of both logitudinal modes gives the free spectral range (FSR) which is 85nm and the ultimate limit for continuous (mode-hop free) wavelength tuning. The maximum λmax and minimum wavelength λmin as well as the tuning range Δλ = λmax − λmin are extracted from the envelopes and plotted in Fig. 4(a). Additionally, the plot shows the centre wavelength of the tuning ranges. As expected, the center wavelength of the tuning range shifts to higher values, since the gain maximum of the active region shifts to higher wavelengths with an increasing device temperature. Besides the red shift of the gain peak, the gain bandwidth decreases with temperature, due to increasing internal losses. Figure 4(b) shows the tuning range over temperature. Surprisingly, the tuning range increases from 85nm at 20°C to a maximum of 92nm at 40°C.
Fig. 4: Minimum λmin, maximum λmax and center wavelength λc of the tuning range over temperature (a). Tuning ranges Δλ over temperature (b).
For both temperatures the continuous tuning range is limited by the FSR. Thus the essential gain bandwidth for laser inversion is still broader then the FSR (above 40°C the decreasing gain bandwidth dominates and the tuning range becomes smaller as the FSR). As a result, the FSR itself must have increased with temperature to increase the tuning range. To increase the FSR, the optical length Lopt of the cavity must have decreased. The FSR can be mathematically expressed by:
at which λc is the centre wavelength of the tuning range. The order q of a longitudinal mode is defined by the resonance equation: Using Eq. (1), (2) with the measured wavelength λq ≈ λc = 1565nm and its corresponding FSR = 85nm, one gets the order q = 18 of the lasing mode. Differentiating Eq. (2) with respect to λq gives the relation between wavelength tuning Δλ and change in cavity length ΔLopt: with a tuning efficiency of 2/q = 2/18 ≈ 0.11. To increase the continuous tuning range (the FSR), the order q has to be reduced by at least one order (or by one FSR), which means Lopt must decrease. To tune over a full FSR (Δλ = FSR) or in other words to reduce q by one, the length of the resonator has to be tuned by ΔL = λq/2. For λq = 1550nm this means that the MEMS-DBR has to move over a distance of ΔL = 775nm. According to Eq. (1) and (2), reducing q = 18 to q = 17 will increase the FSR from 85nm to 92nm.Tracking the peak wavelength of a resonant peak with fixed order q = 18 over temperature, with λ18 = 1554nm at a tuning current of 0mA, shows a linear decrease of the peak wavelength with increasing temperature (see Fig. 5(a)). A linear fit reveals a wavelength shift of dλ18/dT = −4.5nmK−1. Following Eq. (3), this confirms our assumption, that the optical length of the resonator becomes shorter. Additionally it shows, that a temperature change ΔT = 20°C is sufficient to tune over Δλ18 = 90nm > FSR = 85nm. As soon as the peak crosses the lower limit λmin = 1513nm of the tuning range at approx. 29°C, the next lower order peak λ17 starts to lase at the upper limit of the tuning range at approx. λmax = 1605nm. By increasing the temperature to 40°C, the lasing peak λ17 moves to 1555nm, which corresponds to the initial lasing wavelength of the higher order mode λ18. The lower order mode λ17 can now be tuned over 92nm, since its FSR has increased to 92nm as well. The result perfectly fits to our previous calculation. The increase of the FSR is caused by a temperature induced decrease of the air gap length Lair. Whereas the MEMS tuning current ΔIMEMS heats up the MEMS-DBR locally, causing an increasing air gap length and a red shift of the wavelength, a contrary effect is observed if the whole device temperature (of half-VCSEL and MEMS-DBR) is increased. The reason are different coefficients of thermal expansion α. The half-VCSEL substrate mainly consists of Au (the Au substrate has a thickness of 50μm to 60μm) and little InP (≈ 1.5μm) with the coefficients αAu = 14.2 × 10−6 K−1 and αInP = 4.6 × 10−6 K−1 respectively. Whereas the dielectric materials SiO/SiN of the MEMS-DBR have smaller coefficients with αSiO = 0.5 × 10−6 K−1 and αSiN = 2.3 × 10−6 K−1. As a result an increase in temperature of the whole device structure leads to a larger thermal expansion of the substrate 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 as it is schematically illustrated in 5(b).
Fig. 5: Peak wavelength λ18 over device temperature (a). Schematic for change in the resonator geometry with increasing temperature (b). The air gap shrinks with an increasing temperature.
To determine the optical output power over temperature and wavelength, the emission wavelength is tuned to different values within the tuning ranges at different temperatures. To measure the output power for each of these wavelengths and temperatures, the fibre is connected to an optical power metre. For a fixed tuning current and thus a fixed wavelength, as well es a fixed temperature, the laser current is tuned from 0mA to above thermal rollover. Figure 6(a) shows the measured optical output power over the laser current at a fixed wavelength of 1560nm for different device temperatures. Before starting the measurement at a certain temperature, the laser is tuned back to 1560nm with the MEMS-DBR current ΔIMEMS to compensate the wavelength drift caused by a change in device temperature. At 70°C the output power is still above 0.6mW, which outperforms any results of long wavelength MEMS-VCSEL known from literature. Figure 6(b) shows the maximum output power at thermal rollover plotted over device temperature. A linear fit gives a power decrease of 0.046mWK−1 predicting laser operation up to 80°C.
Fig. 6: Output power over VCSEL current at different wafer temperatures (a) and maximum output power over temperature (b). All measurements are taken at a constant wavelength of 1560nm.
Further measurements have been done for fixed laser currents 43mA, 35mA, 30mA and 29mA at device temperatures of 20°C, 40°C, 60°C and 70°C respectively. The laser current needs to be readjusted with the device temperature, since the thermal rollover is shifting to lower current values with an increasing temperature. Figure 7 shows the resulting output power over the emitted wavelength. At 20°C the device reaches an optical peak power > 3mW. Over the entire tuning range, the power does not drop below 1.1mW. For 60°C and 70°C it is > 0.1mW with peak powers of 1.1mW and 0.8mW respectively.
4. Conclusion
In this paper we investigated the temperature dependent tuning characteristics of surface micromachined MEMS-VCSEL. A tuning range of 92nm has been demonstrated at a temperature of 40°C for the first time. The demonstrated tuning range of 56nm at 70°C outperforms similar tuning ranges at 20°C of other MEMS-VCSEL presented in literature. A device specific negative wavelength shift with an increasing temperature of −4.5nmK−1 was investigated and explained. The output power reaches peak values of > 3mW and does not drop below 1.1mW over the entire tuning range at 20°C. With the large temperature operation range, the needed electrical power for active cooling can be reduced (the MEMS-VCSEL does not need to be cooled down to 20°C) so we are confident to meet the criteria of SFP+ with this new MEMS-VCSEL.
Acknowledgments
This work was supported by the Deutsche Forschungsgesellschaft (DFG) within the Graduiertenkolleg TICMO (GRK 1037), the Bundesministerium für Bildung und Forschung (BMBF) within the project VCSEL-TRX and the LOEWE Schwerpunkt Sensors Towards THz.
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