- Opto-Electronic Advances
- Vol. 1, Issue 3, 180005 (2018)
Abstract
Keywords
Background
To meet the demand on the rapid development of the internet at present, versatile laser diodes based optical fiber transmission has been widely applied not only in long-haul backbone (with single-mode fiber, SMF and distributed feedback laser diode, DFBLD) and medium-range metropolitan area networks (with SMF/DFBLD or single-mode vertical cavity surface emitting laser, VCSEL), but also in short-distance area networks or intra/inter data center links (with SMF/DFBLD or multi-mode fiber and multi-mode VCSEL). In recent years, the convergence and streaming with combinations of the broadband voice and video data urge the internet related industry to develop the advanced optical communication solutions which fulfill the huge global market need. All developed and developing countries have established the high-speed optical fiber network infrastructures to achieve both the fiber to the home (FTTH), cloud/mobile data storage and streaming, and the intra/inter data center applications, etc., which is an interdisciplinary integration and construction between wired and wireless communication services. Particularly, owing to the urgent requirement on the higher data transmission rate for the faster up-/down-stream data exchange in intra data centers, the currently available transmission link based on multi-mode VCSEL and multi-mode fiber at 10 Gbps/ channel has gradually found its bottleneck to supply the tremendous network flux instantaneously. Beyond the specification of the 40/100 Gbps data center that has been commercially available, the definition and design of next-generation 400-Gbps or even 1.6-Tbps transceiver module based on high-speed VCSELs at 850 nm has also been initiated toward practical applications no later than 2020.
To catch up the developing pace of huge-amount data streaming in data center for the fusion among wireless mobile, wired telecom, cloud data center, and the FTTH networks in the near future, the investigation on the VCSEL based optical transceiver circuit module with optimized transmission data rate and extended propagation distance in multi-mode fiber become important key technologies. Especially for the required transmission capacity of the cloud transmission within intra- or among inter-data-center networks, more than millions of VCSEL-multi-mode fiber (MMF) links for cloud computing located within super data centers with the least space beyond 100, 000 ft2 and the consumed power above 10 MW, which inevitably lengthens the distance between the server rooms and even among the different severs in the sever room to make the transmission line more complicated. Therefore, each VCSEL based transceiver module has to enhance its transmission rate to 100~400 Gbps to improve the total data transmission capacity in a distance as short as 50~100 m such that the module size, energy consumption and maintaining cost can be concurrently scaled down. Moreover, the fourth-generation intelligent mobile communications (4G) with the wireless transmission rate from 5 to 208 Mbps can transmit the telephone, e-mail and video and other multimedia information. For the fifth-generation intelligent mobile communications (5G), the wireless transmission rate may be achieved to 10 Gbps, which is 100 times larger than that of 4G communication. Therefore, the large capacity and fast transceiver rate of 400 Gbps optical communication provides a key technology to apply to Internet of Things (IoT), mobile networking, super high definition video, and big data. To achieve the requirement of the high-speed and large-capacity to apply to the super computers, cloud computing, and data centers, the VCSEL is developed to become the light source of the short-reach optical links because this laser has some advantages including the low power-consumption, the effectively coupling to fiber, the low threshold current, and the high power conversion efficiency.
Design of the high-speed 850-nm VCSEL
To achieve the high-speed VCSELs, the carriers and photons directly influence the modulation speed of the VCSEL for data transmission. In principle, the relaxation frequency is the natural oscillation frequency between carriers and photons in the VCSEL which can be expressed as below:
where
Figure 1.Schematic plot of intrinsic modulation response with increasing photon densities.
In addition, the second limitation on the modulation bandwidth of the VCSEL is originated from the parasitic resistance and capacitance.
Figure 2.(
To decrease the parasitic capacitance, the multiple oxide layer was proposed to replace the single oxide layer because the device capacitance is inversely proportional to the oxide layer thickness
Figure 3.Simulated frequency responses of the 850-nm VCSEL with (a) single- and (b) double-confined oxide layers.
Figure 4.TEM image of the VCSEL with double-confined oxide layers.
The third limitation to restrict the modulation bandwidth of the 850-nm VCSEL is dominated by the internal heating induced thermal effect in the active layer of the device. Without considering the damping and electrical parasitic effects, the thermal effect induced maximal modulation bandwidth can be estimated as below
where
Mode control of the high-speed 850-nm VCSEL
In the recent researches, the 850-nm multi-mode (MM) VCSELs combining with the MM fiber (MMF) to form the transmission link is one of the cost-effective solutions
The reduction on the transverse modes of the MM VCSELs to improve the data transmission becomes an important issue
Figure 5.Optical spectra of the VSCELs with (a) 6-μm and (b) 10-μm oxide confined apertures.
In addition, the lateral modal distribution for VCSEL with different sizes of the oxide-confined aperture under various bias currents is shown in
Figure 6.Near field lateral modal distributions of the VSCELs with (a) 6-μm and (b) 10-μm oxide confined apertures under different bias current conditions and (c) Cross-sectional SEM image of the VCSEL.
In addition, the small-size oxide-confined aperture increases the current density, which contributes to the improvement of the 3-dB modulation bandwidth (
Figure 7.Frequency responses of the VSCELs with (a) 6-μm and (b) 10-μm oxide confined apertures under different bias current conditions.
However, the oxide confinement method increases the differential resistance of the device, which needs the same current injection carriers under the higher bias, which also induces the impendence mismatch to degrade the data transmission
Figure 8.Optical spectra and frequency responses of the differenttype VCSELs.
In principle, the future research directions to improve the bandwidth of the VCSEL will include the adjustment on RC time constant of the VCSEL by decreasing either its capacitance or resistance. One straightforward way is to decrease the mesa area of the VCSEL, which can effectively reduce the device capacitance. However, this method would concurrently increase the device resistance. Therefore, the increasing doping density is expected to solve the increment on device resistance caused by reducing mesa area. The enhancement of device bandwidth is expectable with aforementioned processes. In addition, another potential approach relies on improving the epitaxial quality during growth, which effectively reduces the defects in the VCSEL so as to decrease the device resistance.
Data transmission performance of the high-speed VCSEL
For practical applications, several studies of the VCSEL for intra-data center links have been reported. In 2013, Westbergh et al. used the VCSEL with the 7-μm oxide-confined aperture to achieve the maximal modulation depth to 27 GHz, which performs the NRZ-OOK data transmission at 44 Gbps over 50-m OM4 fiber and at 47 Gbps under BtB condition
For the NRZ-OOK data transmission, the data rate is mostly dependent on the modulation bandwidth of the VCSEL because of the lower spectral usage efficiency for the NRZ-OOK data transmission. To solve this problem, the versatile data formats have been developed. The 4-level pulse amplitude modulation (PAM-4) data format is one of solutions to approach for decoding the VCSEL because the half of modulation bandwidth for the PAM-4 data format is used to achieve the same data rate as compared to the NRZ-OOK data format
Moreover, a pre-emphasis filter technology for the PAM-4 data format was proposed to apply to VCSEL, which further improves the data rate to 100 Gbps over 100-m MM fiber
Figure 9.BERs of the SM VCSEL chip carried and data waveform pre-emphasized PAM-4 data at different bandwidths after BtB, 100-m, 200-m, and 300-m OM4 MMF transmissions with the corresponding eye-diagram.
To further utilize the bandwidth of laser, the quadrature amplitude modulation orthogonal frequency division multiplexing (QAM-OFDM) was proposed to solve this problem
Moreover, Kao et al. observed the VCSEL with different transverse modes to process the 16-QAM-OFDM data format over 100-m OM4 fiber transmission
Figure 10.BER of the 100-m OM4-MM fiber transmitted 16-QAMOFDM data for SM VCSEL under the different receiving powers.
Later, the same group utilized the power of the low-frequency OFDM subcarrier to compensate the power of the high-frequency OFDM subcarriers by the pre-leveling technology, which improves the transmission performance of the 100-m OM4 fiber transmitted 16-QAM-OFDM data for the FM VCSEL with the modulation bandwidth of 22 GHz
Figure 11.(
Other applications
With the great advanced development of information technology in future high-speed internet, the applications and related markets, such as the 5G mobile networks and cloud implementation using smart service systems have become increasingly widespread. Attributed to the expansion of intelligent applications, smart cities/homes can be viewed as a complex, intelligent system that allows all kinds of electronic devices with intelligent monitoring and information transmission capabilities to interact with each other through communication technologies. To build a smart city, the sensing client is one of the most important links in the intelligent service system. For example, the major motor vehicle manufacturers deploy sensors, such as light detection and ranging (LiDAR) and depth-sensing lenses to achieve the goal of fully autonomous vehicles. LiDAR is an optical telemetry technology. When the optical radar irradiates a laser beam on the target, the distance between the target and the optical radar is measured according to the time of flight of light transmitted and received, and the position information of the target is estimated from the emission and reception angles of the laser light. To date, the direction of laser beam for a sensing system has principally been governed by an external mechanical system that incorporates macroscopic-size optical components including double-swing mirror, galvanometer-driven mirrors and polygon reflecting prism. The traditional sensor-mechanical LiDAR has many disadvantages, e.g., large size, expensive, heavy, high power consumption, low performance, low reliability, and slow scanning speed. The field of view (FOV) of LiDAR is related to beam divergence, the primary beam angle should be in range of 0.1 to 1 milliradian, and small FOV can be used for detailed local mapping, edge detection, and detailed vegetation canopy studies. Large FOV can be used for more complete ground sampling and more interactions with multiple vertical structures.
To achieve the near-infrared laser with the capability of an optical radar component, the laser device must be able to flexibly regulate the emission angle that allows active beam steering. Prof. F. Koyama's group from Tokyo Institute of Technology proposed that the laser beam steering can be successfully achieved by using GaAs-based VCSELs combined with a slow-light Bragg reflector waveguide
In 2013, Prof. F. Koyama proposed a high-resolution beam-steering device based on a vertical-cavity surface-emitting laser with a 1-mm-long active-type Bragg reflector waveguide
In addition, an optical phased array (OPA) with multiple optical antenna elements may provide equal-intensity coherent signals by modulating the phase of individual antenna. In 2013, J. Sun et al., at Massachusetts Institute of Technology (MIT) demonstrated a large-scale nanophotonic phased array to achieve beam steering built on a silicon photonic platform
Figure 12.(
In 2015, J. C. Hulme, et al. from University of California, Santa Barbara presented the first fully-integrated two-dimensional beam-steering chip on the photonic integrated circuit (PIC) platform
Conclusions
With the development of VCSEL and application requirements, VCSEL not only played an important role of the high-speed and large-capacity to be applied to the supercomputers, cloud computing, 5G communications, and data centers, also applied to face recognition, light detection and ranging (LiDAR) and VR (virtual reality)/AR (augmented reality)/MR (mixed reality) and so on. Through the increasing diversification of market demand for VCSEL, commercial technology companies and research institutions are following the trend of in-depth research, optimizing VCSEL performance and improving output power. Believed that in the near future, VCSEL will have a better development.
Acknowledgements
The authors would like to thank Prof. K Iga and Prof. F Koyama of Tokyo Institute of Technology, Prof. N. Holonyak Jr. and Prof. M. Feng of UIUC, Prof. C. Chang-Hasnain of UC Berkeley, and Prof. S. C. Wang and Prof. T. C. Lu of Chiao Tung University for their insightful suggestions and discussions during the development of GaAs-based VCSEL technology, Crosslight for providing simulation modeling calculate technical support, and Picosun for providing atomic layer deposition technical support. This work was supported by the Ministry of Science and Technology, Taiwan, China (Grants No. MOST 104-2221-E-002-117-MY3, MOST 106-2221- E-002-152-MY3 and MOST 106-2218-E-005-001-) and Excellent Research Projects of Taiwan University (Grant No. NTU-ERP-105R89081).
Competing interests
The authors declare no competing financial interests.
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