High-Speed 1030 nm Anti-Waveguide VCSELs With 25 GHz Modulation Bandwidth

We present the design, fabrication, and performance of high-speed oxide-confined 1030 nm vertical-cavity surface-emitting lasers (VCSELs) with a short anti–waveguiding optical cavity. By using a half-wavelength cavity to enhance the limit of the optical field, the anti-waveguide design increases the oscillator strength, thus promoting high-speed modulation. We carefully optimized the multiple quantum wells and the doping profile in order to achieve high-speed operation. The developed VCSELs exhibit a modulation bandwidth exceeding 25.1 GHz at 25 °C, supporting back-to-back data rates up to 40 Gb/s under binary non-return-to-zero (NRZ) modulation.


I. INTRODUCTION
T HE short-reach optical links based on 850 nm GaAs-based VCSELs over up to 100 m multimode fibers (MMF) have been widely deployed in large scale data centers [1], [2], [3]. However, the high chromatic dispersion of fiber and attenuation at this wavelength are important factors limiting transmission distance [4], [5]. At 850 nm, the fiber chromatic dispersion is as high as ∼85 ps/nm/km and the attenuation exceeds 2 dB/km [6], [7]. With data centers growing in size, the increased demand for longer reach optical interconnects (up to 2 km in large scale data centers [5]) drives the development of longer wavelength VCSELs so that the reach limitations of the 850 nm multimode links can be overcome.
The ideal wavelength is 1310 nm where chromatic dispersion is near zero and the attenuation of MMF is only ∼0.4 dB/km [6], [7]. Up to now, 1310 nm VCSELs have been developed based on quantum-dot [8] and InP material [9], [10]. However, the GaAs-based VCSEL technology, which is superior to the InP-based in terms of speed, efficiency, manufacturability and cost-efficiency, can only be extended to ∼1100 nm using conventional compound semiconductors without compromising reliability [11]. This has created an interest in GaAs-based VCSELs at wavelengths just below 1100 nm where the fiber chromatic dispersion is ∼30 ps/nm/km and the attenuation is below 1 dB/km [6], [7]. a large improvement with respect to 850 nm.
For the purpose of extending the wavelength without compromised reliability [11], [12]. GaAs-based VCSEL with wavelength ∼1100 nm was developed. Using the strained In-GaAs/GaAs quantum wells (QWs) with doped barriers for high differential gain and reduced gain compression, oxide-confined VCSELs at 1090 nm with a modulation bandwidth of 20 GHz were demonstrated in 2006 [13]. Then a bottom-emitting VC-SEL with a bandwidth of 18 GHz using multiple oxide apertures for low capacitance were demonstrated [14]. Furukawa demonstrated 20 GHz bandwidth oxide confined 1060 nm VCSELs using double intra-cavity structure and a dielectric top distributed Bragg reflector (DBR) [15], [16], [17], [18]. Recently, 1060 nm VCSELs utilizing strained InGaAs/GaAsP QWs and multiple oxide apertures with a bandwidth of 22 GHz were realized with similar techniques of commercial VCSELs [19], [20], [21].
An effective way to improve the performance of VCSEL devices is to apply an anti-waveguiding design [22], where the cavity region has a smaller refractive index compared to the average refractive index of a DBR. The anti-waveguiding VCSEL design aimed to suppress the highly undesirable in-plane fundamental mode, which is very important to avoid gain depletion and enhance the oscillator strength for the vertical transition. In this article, we demonstrate the use of an anti-waveguide structure design in the 1030 nm VCSEL. The optimized anti-waveguide structure exhibits better in plane mode suppression effects than traditional anti-waveguide structures. The modulation bandwidth of the VCSEL is 25.1 GHz and the data transmission at rates up to 40 Gb/s is demonstrated at 25°C. The design and fabrication of the VCSEL are presented in Section II and their static characteristics are presented in Section III. Section IV is devoted to small-signal modulation. Section V is devoted to the large-signal modulation and a conclusion is presented in Section VI.

II. VCSEL DESIGN AND FABRICATION
The 1030 nm VCSEL epitaxy material was grown by metalorganic chemical vapor deposition. The first 26 pairs bottom DBR are made of AlAs/GaAs to reduce the thermal impedance. All other mirrors are made of Al 0.9 Ga 0.1 As/GaAs. The DBRs are with graded interfaces and modulation doping for the low resistance and internal optical loss. Multiple oxide apertures are placed in the p-DBR just above the active region. For high longitudinal optical confinement, we use a half-wavelength (λ/2) thick cavity. The active region is positioned in a thin separate confinement heterostructure at the center of λ/2 cavity. We have adopted the anti-waveguide design. Compared with the average refractive index of DBR, the refractive index of the cavity area is lower. The design of VCSELs with antiwaveguide cavities is an effective way to achieve high modulation bandwidth, which is broadly used by the industry [23]. Fig. 1(a) shows the vertical profiles of the refractive index, of the vertical cavity mode (red), and of the in-plane mode (blue) for a conventional anti-waveguide λ/2-cavity device. The in-plane mode electric field intensity in the active region is approximately the same as the vertical cavity mode. This will lead to the parasitism of in-plane laser, thus reducing the intensity of vertical oscillation. Fig. 1(b) shows our design of a λ/2-cavity device. The structures in both top/bottom DBRs and the λ/2 cavity were carefully designed to shift the fundamental in-plane mode from the cavity region into the DBR. The electric field strength (|E|) of the in-plane mode in the active region is about 9 times lower than that of the vertical mode. Furthermore, the in-plane mode has significant losses due to leakage into the top DBR, and the oscillator strength of the vertical mode is enhanced compared to the conventional anti-waveguide λ/2-cavity VCSEL, which supports a high modulation bandwidth.
In the active region, we use partially strain-compensated InGaAs/GaAsP QWs, with net compressive strain. Above the active region is a p-DBR stack with 2 pairs of Al 0.98 Ga 0.02 As/GaAs, 4 pairs of Al 0.96 Ga 0.04 As/GaAs, 14 pairs of Al 0.9 Ga 0.1 As/GaAs and finally a GaAs contact layer. Transverse optical and current confinement is mainly provided by two Al 0.98 Ga 0.02 As layers after wet oxidation. The four Al 0.96 Ga 0.04 As layers are used to reduce the parasitic capacitance of oxide apertures.
The fabrication process starts with a 22-μm diameter DBR mesa defined by inductively coupled plasma dry etching. A laser interferometry apparatus was used to monitor the etching. Right after the dry etching, the sample was sent to an oxidation furnace filled with H 2 O vapor and N 2 carrier gas to laterally oxidize the high Al-composition layers in the top p-DBR, the aperture and parasitic reduction oxide layers. Side view scanning electron microscopy (SEM) images of the oxide layers are shown in Fig. 2(b). The p-type Ti/Pt/Au and n-type Ni/AuGe/Ni/Au metal contact was evaporated and annealed, respectively. The device fabrication is finished with benzocyclobutene planarization, via-hole etching, and Ti/Au metal interconnect evaporation. Schematic cross-section of the VCSEL structure is shown in Fig. 2(a).

III. STATIC CHARACTERISTICS
The measured light-current-voltage (L-I-V) characteristics of the VCSELs with oxide-aperture diameters of ∼5 μm at different temperature are shown in Fig. 3. From the L-I-V measurements, we extract the threshold currents depending on temperature. This data is presented in Fig. 4. The threshold current increases from I th = 0.53 mA at 25°C to 0.76 mA at 45°C, to 1.05 mA at 65°C, and finally to 1.48 mA at 85°C. At higher temperatures, the threshold current increases due to the increased optical absorption, reduced internal efficiency, and an increase of the gain-wavelength detuning (lower gain), and is close to twice the 25°C value at 85°C. The minimum threshold current for this device is below 25°C.

IV. SMALL-SIGNAL MODULATION
The small-signal modulation response (S 21 ) can be modelled as a second order damped system described by a resonance frequency f r and a damping rate γ [24]. Accounting for both the intrinsic response of the laser and the extrinsic, parasitic response, giving the full three-pole transfer function where A is an amplitude factor, f is the modulation frequency, f p is the parasitic cut-off frequency. The parasitic transfer function may also be determined by S 11 measurements, which also allow fitting values of the parasitic elements to an equivalent circuit model [25].
The resonance frequency increases linearly with the square root of bias current above threshold according to (2) With where η i is the internal quantum efficiency, Γ is the optical confinement factor, ν g the group velocity, L a the total thickness of the QWs, g/n is the differential gain, and χ the transport factor. VCSELs with a large D-factor and low threshold current reach high resonance frequencies at low bias currents, which enables energy-efficient high-speed modulation. A large D-factor is also important in order to reach a high maximal VCSEL bandwidth, equivalent to reaching a large resonance frequency before current induced self-heating and thermal saturation deteriorates performance. The damping is increasing linearly with the square of the resonance frequency and determines an intrinsic upper limit to the modulation bandwidth. The rate at which damping increases is quantified by the K-factor With where ε is the gain compression factor, τ p is the photon lifetime and the offset γ 0 is approximately equal to the inverse differential carrier lifetime [24]. The small signal modulation response was measured using a 67 GHz vector network analyzer (Keysight) connected to the VCSEL under test through a high-speed bias-T and a high-speed RF probe (Picoprobe). The VCSELs were probed directly on wafer and the light was coupled into an angled multimode fiber and then was fed to a 30 GHz photodetector (Thorlabs) via a variable optical attenuator (VOA) to avoid saturation of the detector. The measured data was corrected for the limited frequency response of the probe and detector. Fig. 6(a) and (b) show the small signal modulation response (S 21 ) at 25°C and 85°C, respectively, and plotted together with fits to a three-pole transfer function (1). The low frequency roll-off which is most pronounced at low bias in Fig. 6(a) and (b) is attributed to excessive damping from spatial hole burning [26]. The 5 μm aperture VCSEL reaches a maximum bandwidth of 25.1 GHz and 20.6 GHz at 12 mA under 25°C and 85°C, respectively. There are more significant 3dB drops in S21 at higher bias current, which can be attributed to the thermal rollover of f r under high bias current.
A three-pole transfer function (1) was fitted to the measured data to extract the resonance frequency and the damping rate. By plotting of fr against √ I − I th , the D-factor of the VCSEL can be extracted. Fig. 7 shows the f r plotted against the square root of bias current (I) above threshold (I th ) under different temperature conditions. The extracted D-factor is 7.8 GHz/mA 1/2 and the f r saturates at a maximum value of ∼22 GHz at 25°C. Increasing the temperature to 85°C only results in a minor reduction of the D-factor to 7.2 GHz/mA 1/2 , but since thermal saturation sets in earlier at this temperature, the maximum resonance frequency is reduced more noticeably to ∼19 GHz. The D-factor is relatively temperature stable, dropping only 14% from 25 to 85°C. The D-factor is smaller for higher operating temperatures. The reason is that the internal quantum efficiency decreases with the increase of temperature. K-factors were extracted from linear fits of (4) as seen in Fig. 8. The K-factors were extracted from the low current regime, before thermal saturation begins, and were  found to be relatively temperature insensitive at 0.243-0.209 ns from 25 to 85°C, showing a relatively weak temperature dependence. This can be explained by the differential gain change with temperature. The differential gain of the quantum well will increase slightly with the increase of temperature. It can be seen from (5) that the K factor decreases with the increase of differential gain.

V. LARGE-SIGNAL MODULATION
To validate the device's data-transmitting capability, we perform the eye diagram and bit-error-rate (BER) measurement. The transmission interconnect setup consists of a SHF Bit Pattern Generator (BPG) that provides the modulation bit sequence, the same light collimation module used for DC and RF measurements, a 2 m and 100m OM5 optical fiber that collects the coupled light from the light collimation module, and a Thorlabs 30 GHz photoreceiver that converts the collected optical signal back into electrical signal. The test bit sequence used is a non-return-to-zero 2 7 -1 by the SHF BPG. The converted electrical signal from the photoreceiver is sent to the SHF Error Analyzer for BER testing. A Tektronix Oscilloscope with a 59 GHz bandwidth sampling module is used to capture the eye diagrams. Fig. 9 shows the results of the back-to-back (BTB, ∼2 m OM5 MMF) and 100 m (OM5 MMF) transmission experiments at 25°C. At I = 10 mA, the VCSEL is able to demonstrate transmission at 40 Gb/s (BTB) and 35 Gb/s (100 m). The eyes are clearly open and BER <10 −12 is reached in both cases. The VCSEL also has a large RMS spectral width (>0.6 nm) and because of modal fiber dispersion, it is unable to achieve longer distance (>100 m) transmission.

VI. CONCLUSION
In conclusion, we have demonstrated a high-speed 1030 nm VCSELs with a short optical cavity and multiple oxide apertures. The use of a short λ/2 cavity improves the longitudinal confinement factor and the dynamic properties. To reduce capacitance, we use multiple oxide apertures with two primary apertures for transverse optical and current confinement and four secondary oxidizable AlGaAs layers for parasitic capacitance reduction. Applying an anti-waveguiding cavity leads to a strong suppression of parasitic in-plane optical modes, which supports a high modulation bandwidth. The 1030 nm VCSEL with ∼5 μm aperture demonstrated a bandwidth of 25.1 GHz at 25°C, and can be modulated for error-free data transmission at rates up to 40 Gb/s. Our device is promising to be used as high-speed transmitters in the next generation data centers.