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Abstract
Dilute nitride (DN) vertical cavity surface emitting lasers (VCSELs) emitting near 1300 nm exhibit state-of-the-art performance including bandwidths of 10 GHz and a record high error-free data transmission of 12 Gbps. Renewed interest in DN VCSELs stems from emerging applications in kilometer-reach digital communication across optical fiber and across free space via eye safe beams, time-of-flight and structured light sensing, and photonic-electronic integrated circuit optical interconnects. We produce VCSEL wafers in a production molecular beam epitaxy system on 3- and 4-inch diameter GaAs wafers. We report record dynamic performance for our test VCSELs with oxide aperture diameters ranging from 2 to 12 µm.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Following the pioneering work on dilute nitride semiconductor material [1] and dilute nitride laser diodes [2], several groups - circa 1998 to 2003 [3–8] - demonstrated dilute nitride vertical cavity surface emitting lasers (VCSELs) emitting light at ∼1300 nm. A significant benefit of dilute nitride (DN) VCSELs is they may be composed (almost) of the same III-V epitaxial material layers commonly used for 780-1060 nm VCSELs – i.e. AlGaAs distributed Bragg reflectors (DBRs) on GaAs substrates. The key difference (to create a dilute nitride VCSEL) is the addition of a small percentage of nitrogen (N) and optionally also antimony (Sb) to the GaAs or GaInAs quantum well layer or layers – along with well-known corresponding thickness increases of the DBR layers and the optical cavity to set the resonant wavelength at ∼1300 nm. Alternative 1300 nm VCSEL approaches – for example – include InP-based active regions with a buried tunnel junction (TJ) and dielectric DBRs [9], and oxide aperture VCSELs with InAs quantum dot active regions grown on GaAs substrates with AlGaAs DBRs [10].
Since the early DN VCSEL work, no significant improvements have been reported for DN VCSELs excepting the work in [11] and [12]. Three reasons for the stasis in DN VCSEL research and development include: 1) the digital optical communication market was well served by low cost and reliable 850 nm VCSELs; 2) the highly non-trivial growth conditions required to produce high quality and highly uniform DN quantum well (QW) active regions, especially in large commercial epitaxial machines; and 3) the preponderance and ready availability of InP-based edge-emitting laser diodes emitting at 1310 and 1550 nm for medium and long distance optical telecommunication.
Renewed interest in dilute nitride VCSELs arises (we believe) from: 1) the need for low optical output power on-chip laser diodes at 1200-1600 nm for applications in GaAs-based photonic-electronic integrated circuits; 2) the growing market for eye safe optical sensing systems based on VCSELs and VCSEL arrays; and 3) the emerging market for low cost optical links based on single-mode optical fiber at 1310 nm for link distances exceeding 1 km. Other potentially lucrative markets for DN VCSELs include light sources for 5G (Fifth Generation) mobile and optical wireless applications for example: 1) free-space (last mile or indoor personal network) optical communication; 2) the Internet of Things; 3) low cost light detection and ranging systems; and 4) bio-medical sensing/diagnostic devices and systems. Given the rekindled desire for longer wavelength VCSELs suitable for sensing and GaAs photonic integrated circuits we investigate 1300 nm DN VCSELs with our advanced high frequency (HF) device geometry and achieve a 10 GHz bandwidth and record 12 gigabit-per-second (Gbps) data transmission rate.
2. Epitaxial design
Our experimental VCSELs are grown in a commercial multiple wafer molecular beam epitaxy machine on three-inch diameter GaAs wafers. We investigate an epitaxial design targeted as a generic baseline (test) structure that may be used as a reference for more advanced future DN VCSEL designs. As illustrated in Fig. 1, on standard (001) surface-oriented (n+)GaAs substrates we grow a 37.5 period silicon-doped bottom AlxGa1-xAs DBR with x = 0.0 (high refractive index) and x = 0.9 (low refractive index) layers plus 27 nm thick inter-DBR step-graded regions, followed by a 1λ (optically thick) optical cavity containing three 8 nm thick GaInNAsSb quantum wells (QWs), 10 nm thick GaAs barrier layers separating the QWs, and doped GaAs spacer layers. The optical cavity is followed by an 18 period carbon-doped top AlxGa1-xAs DBR that is compositionally identical to the n-doped bottom DBR, where the first low index p-DBR layer adjacent to the optical cavity includes a 23 nm thick x = 0.98 layer. This Al-rich layer will later serve as a current aperture and mode confinement layer after selective thermal oxidation at 420 °C at 50 mbar in a saturated H20+N2 environment. We cleave the starting wafers into four quarter pieces and process double mesa VCSELs.
Fig. 1. Cross-section (left) of half of a 1300 nm DN VCSEL via a collage including a scanning electron microscopy image, added text, and an illustrated top p-metal contact, and (right) simulated 1D real index (blue line) and optical field intensity on resonance (red line) profiles.
3. Results
In Fig. 2 we show the measured continuous wave (CW) light optical output power-current-voltage (LIV) characteristics of our DN VCSELs with oxide aperture diameters (ϕ) ranging from ϕ∼2 to 12 µm, taken at a controlled platen temperature of 25 °C. The threshold current increases from ∼1 mA to well over 7 mA with increasing ϕ, while the maximum LI slope is ∼0.12 W/A just past threshold and the differential series resistance near LI rollover decreases from about 450 to below 100 Ω as ϕ increases. The maximum L is ∼0.8 mW for the VCSEL with ϕ∼7 µm then generally decreases with further increases in ϕ indicating likely increasing cavity losses, saturating QW gain, and an increase in the wavelength mismatch of the cavity etalon resonance wavelength and the QW gain peak wavelength due to heating. In Fig. 3 we show the LIV characteristics of a single DN VCSEL with ϕ∼3 µm for controlled probe station temperatures of 15 to 55 °C in 10 °C steps, with the corresponding emission spectra given in Fig. 4. The quasi-single mode emission with a side-mode-suppression-ratio of just above 35 dB has a fundamental mode LP01 that varies with temperature as 0.085 nm/°C.
Fig. 2. Static LIV characteristics at room temperature (25 °C) for 1300 nm DN VCSELs with oxide aperture diameters ranging from 2 to 12 µm (left) and corresponding differential series resistance versus bias current (right).
Fig. 3. Continuous wave (CW) optical output power and voltage versus bias current for a DN VCSEL with ϕ ∼3 µm at 15 to 55 °C in 10 °C steps.
Fig. 4. Continuous wave emission spectra of a ϕ ∼3 µm DN VCSEL at bias current of 3 mA taken at heatsink temperatures from 15 to 55 °C in 10 °C steps.
In Fig. 5 we show the measured small-signal modulation frequency response and corresponding scattering parameter magnitude |S21| curve fits for a 3 µm DN VCSEL at 25 °C at I = 2, 3, and 5 mA. The −3 dB bandwidth (f3dB) has a record maximum value of 10 GHz. The maximum values of f3dB vary with temperature, as shown in Fig. 6 where the maximum f3dB is 11 GHz at 15 °C and about 8 GHz at 35 °C. We next perform a data transmission measurement across about 0.5 m of OM1 optical multiple-mode fiber using a nonreturn to zero 2-level pulse amplitude modulation and a pseudorandom binary sequence of word length 27−1. The BER test results including optical eyes are given in Fig. 7, where we achieve error-free data transmission (defined to be a BER < 1 × 10−12) at bit rates of 10 and 12 Gbps. The relationship between the shape of the |S21| response and the quality of optical eyes in data transmission tests is well described in [13] – thus leaving plenty of room to optimize our baseline DN VCSEL design for future optical communication applications. We summarize our results in Table 1 and include historical DN VCSEL data for comparison.
Fig. 5. Small signal modulation frequency response for an ϕ ∼3 µm DN VCSEL at bias currents of 2, 3, and 5 mA.
Fig. 6. Small-signal modulation bandwidth frequency (f3dB) versus bias current extracted from the |S21| response curves for the ϕ ∼3 µm DN VCSEL taken at heatsink T = 15, 25, and 35 °C.
Fig. 7. Bit error ratio (BER) versus received optical power at 10 and 12 Gbps at 25 °C, and optical eye patterns at the minimum recorded BERs.
Table 1. Selected historical performance of dilute nitride VCSELs
4. Summary
Due to the need for medium to long-reach digital data communication across optical fiber (for example, single mode fiber using low cost 1300 nm VCSELs), interest in eye safe beams for sensing and free space communication (via for example 2D 1300 nm VCSEL arrays), and interest in 1300 nm VCSELs suitable for integration with GaAs-based photonic integrated circuits, we note a resurgence of interest in dilute nitride (DN) VCSELs. Via a production MBE machine and an advanced university planar VCSEL processing scheme that uses a high frequency co-planar GSG mask set, we demonstrated DN VCSELs with a bandwidth of 10 GHz and record error free (BER < 1 × 10−12) 12 Gbps bit rates across a standard OM1 multiple mode fiber patch cord. The critical aspect for continued DN VCSEL development – seeking to match the performance of 850 to 1060 nm GaAs-based VCSELs and 1300 nm InP-based VCSELs – we believe – is experiments focused on the exact epitaxial growth conditions and in situ or post growth annealing steps.
Disclosures
The authors declare no conflicts of interest.
References
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