VCSELs at OFC2024

Here are some articles on VCSELs from OFC2024 that I've organized:

1.The VCSEL structure from National Taiwan University is as follows: The key points include the generation of a single transverse mode by further optimizing the zinc diffusion and oxide aperture structure, avoiding spatial hole burning effects, and simultaneously optimizing bandwidth. The active layer consists of 4 sets of In0.07Ga0.93As/Al0.3Ga0.7As multiple quantum wells (MQWs), while the distributed Bragg reflector (DBR) comprises 39 pairs of n-type and 24 pairs of p-type Al0.93Ga0.07As/Al0.15Ga0.85As. The oxide layer is 25 nm thick Al0.98Ga0.02As. The final device achieves high bandwidth (27 GHz), high power (6.7 mW), low relative intensity noise (RIN) (-137 dB/Hz), and maintains a 56 Gbps eye diagram with -6 dB strong optical feedback, as well as achieving error-free, non-equalized transmission at 46 Gbps over 0.5 km multimode fiber (MMF).

The figure below illustrates the comparison between the results from this study and the literature. Due to its high single-mode output power (6.7 mW @ 9 mA) and high-speed performance, this VCSEL achieves the longest distance at the highest bit rate among all reported high-speed 850 nm VCSELs under similar conditions, without requiring the use of equalizers.

2.Broadcom's VCSEL technology, based on oxide-constrained VCSELs, can support 800 Gbps (100 Gbps/lane) PAM4 links. The figure below illustrates the evolution of Broadcom's 850nm VCSEL modulation rate over the past 25 years since 1997. It indicates an exponential growth in modulation rate over time, roughly doubling every four years. The latest 802.3db standard defines the physical layer specifications for multi-mode fiber (MMF) links, with a 100m OM4 link operating at a channel rate of 100 Gb/s. However, extending the modulation rate to 200 Gb/s while ensuring reliability in practical applications poses challenges such as increasing optical bandwidth and improving VCSEL parasitics performance.

For a 200G link, considering the decrease in receiver sensitivity, a high optical modulation amplitude (OMA) is required, thus continuing to use multimode VCSELs. The bottom left figure shows the 100 GBd PAM4 eye diagram of an 850 nm VCSEL with a extinction ratio of 2.1 dB. The lateral (time) tilt of the three sub-eyes in the optical eye diagram is caused by the nonlinear response of the directly modulated VCSEL. The bottom right figure displays the 106.25 Gb/s NRZ eye diagram of an 850 nm VCSEL, with an extinction ratio of 2.1 dB, and a clear eye diagram is observed after equalization.

3.Coherent has achieved an optical aperture 940 nm 100G VCSEL with a bandwidth exceeding 29 GHz. Previously, Coherent had achieved an 850 nm oxide aperture VCSEL with a bandwidth of 27 GHz. To date, the highest reported VCSEL bandwidths at wavelengths of 980 nm and 850 nm are 35 GHz and 30 GHz, respectively. For applications requiring 200 Gbps/lane, bandwidths exceeding 40 GHz are expected. Shrinking the aperture size and mode volume are the main strategies to increase bandwidth for VCSEL devices.

Challenges in achieving oxide apertures with diameters smaller than 3 micrometers include design constraints imposed by the fabrication process and thermal effects on performance, as well as difficulties in achieving high reliability and repeatability in mass production. Therefore, new technologies such as lithographic apertures may be necessary to achieve high bandwidths.

The fabrication process involves epitaxial growth of the n-distributed Bragg reflector (DBR), multiple quantum wells (MQWs), λ/2 cavity, and highly doped tunnel junctions. Subsequently, lithographic patterning is used to etch the tunnel region to define the blocking region and the aperture region. Finally, the p-DBR is grown.

The diagram below illustrates a cross-section schematic. The right graph displays the optical current characteristics of a VCSEL with a 4 µm aperture, showing a maximum output power of 4.3 mW and a slope efficiency of 0.6 W/A at 25°C.

The figure below depicts the high-frequency characteristics. At 25°C with a bias of 7 mA, the bandwidth is 29.5 GHz. At 85°C, the maximum modulation bandwidth is 25 GHz with a bias of 6 mA. Testing reveals a parasitic capacitance of 60-70 fF, approximately half that of similarly sized oxide-aperture VCSELs. Figure c illustrates the NRZ eye diagram at 53.125 GBaud, with an extinction ratio of 4 dB at 5 mA.

TRUMPF Photonic demonstrated 56G PAM4 transmission over 500 m of optimized multimode fiber (MMF) using a 980 nm multimode VCSEL. Currently, MMF systems utilize 850 nm VCSELs. However, as data rates exceed 100G, the MMF dispersion (approximately -95 ps/(nm·km)) at 850 nm may become a limiting factor. One solution is to optimize for higher bandwidth using longer wavelengths, such as 980 nm.

In longer wavelength systems, fiber dispersion and attenuation are significantly reduced. Additionally, 980 nm VCSELs offer advantages such as higher gain, temperature stability over a wide range, and robustness. However, at 980 nm, the modal bandwidths of traditional OM3 and OM4 fibers are only 0.91 GHz·km and 1.28 GHz·km, respectively.

To achieve longer transmission distances, the article optimized the MMF by shifting the peak wavelength bandwidth around 980 nm. Ultimately, while meeting IEEE standards, the maximum transmission distance at 980 nm was extended to 500 m, with a measured modal bandwidth of 14.2 GHz·km. The low TDECQ values provide room for link optimization.

5.The VCSELs developed at the University of Illinois are primarily applied in low-temperature supercomputing. The challenge in low-temperature supercomputing lies in the requirement for SFQ (Single Flux Quantum) superconducting circuits to operate at liquid helium temperatures, making traditional high-speed data communication challenging. In comparison to traditional electrical links, optical fiber interconnects demonstrate significant advantages, particularly in reducing frequency-dependent signal attenuation and negligible thermal leakage.

There is a growing demand for interconnect solutions that are both energy-efficient and capable of high-speed data transmission between 4K (liquid helium temperature) and room temperature (RT) electronic devices. Cryo VCSELs serve as key transmitters in low-temperature optical links. They have reported the first wafer-level probing measurements of semiconductor lasers below the temperature of liquid helium.

At a cryogenic temperature of 2.8 K, the threshold current of the 6 µm aperture Cryo-VCSEL decreases to 0.2 mA, while the maximum optical power output reaches 14.1 mW. Despite the continuous increase in laser output power, the optical wavelength and mode remain nearly unchanged due to superior thermal performance.

Through more than 30 thermal cycles from 300 K to 2.8 K and stress tests with Ibias up to 50 mA, the laser threshold and output power performance show no degradation. At bias currents exceeding 7 mA, it achieves a 3 dB bandwidth exceeding 50 GHz.

Using wafer-level probing techniques, Cryo-VCSEL achieves eye diagrams for transmitting 64 Gbps NRZ and up to 128 Gbps PAM-4 data at 2.8 K to room temperature optical receivers.

Here are the references:

【1】Single-Mode VCSEL with Zn-Diffusion Apertures and StrongImmunity Against Optical Feedback for Improved DataTransmission

【2】Toward 200G per Lane VCSEL-based Multimode Links

【3】Cryogenic Oxide-VCSEL at 2.8 K Demonstrates RecordBandwidth f-3dB > 50 GHz, Pout > 14 mW and PAM-4 DataRate up to 128 Gb/s

【4】Lithographic Aperture VCSELs Enabling Beyond 100GDatacom Applications

【5】56G VCSEL Transmission at 980 nm across 500 mMultimode Fiber

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VCSELs at OFC2024