| 1. |
- Haglund, Erik, 1985, et al.
(författare)
-
850 nm datacom VCSELs for higher-speed and longer-reach transmission
- 2013
-
Ingår i: European VCSEL Day 2013.
-
Konferensbidrag (refereegranskat)abstract
- The 850 nm GaAs-based VCSEL is already the dominating technology for transmitters in optical interconnects up to 100 m in datacenters, thanks to low-cost fabrication, excellent high-speed properties at low currents and the existence of high-speed OM4 multimode fiber optimized for this particular wavelength. Future datacenters will require faster and more energy-efficient VCSELs to increase the overall bandwidth and reduce the power consumption of the datacenter network. In addition, longer-reach interconnects exceeding 1 km will also be required as datacenters grow into large multi-building complexes.By optimizing the doping profiles of the DBRs to reduce resistance, using a short (½-λ) cavity to improve longitudinal optical confinement and optimizing the photon lifetime for optimal damping, we obtained a record-high small-signal modulation bandwidth of 28 GHz for a ~4 µm oxide aperture VCSEL. A 7 µm oxide aperture VCSEL (~27 GHz bandwidth) enabled error-free transmission (bit-error-rate 300 m), the large spectral width of VCSELs leads to severe signal degradation by fiber dispersion. We have investigated two methods of fabricating low-spectral width quasi-single mode VCSELs to mitigate this problem. By using a small oxide aperture of ~3 µm, error-free transmission was achieved at 22 Gbit/s over 1.1 km of OM4 fiber. An alternative approach is to use an integrated mode filter in the form of a shallow surface relief to reduce the spectral width of the VCSEL. The mode filter allows for the use of a larger oxide aperture and thereby enables a lower resistance and operation at a lower current density. A 5 µm oxide aperture VCSEL with a mode filter enabled error-free transmission at 25 Gbit/s over 500 m of OM4 fiber.
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| 2. |
- Haglund, Erik, 1985, et al.
(författare)
-
High-speed lasers for optical interconnects
- 2012
-
Ingår i: Swedish Optics and Photonics Days 2012.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In the era of cloud computing, with services such as video streaming, social networking, and online storage and file sharing, the demand for online data processing and storage capacity is growing rapidly. These services are hosted in huge data centers that need not only fast servers but also fast communication between servers. Because copper cables have high attenuation at high frequencies, the most promising solution is to use fiber optical links (called optical interconnects) to connect different parts of the data center. Today, gallium arsenide-based vertical-cavity surface-emitting lasers (VCSELs) emitting at 850 nm are the standard light source in transmitters used in commercially available optical interconnects, operating at up to 14 Gbit/s with a link length of up to 300 m. These lasers have the advantages of low power consumption, fast direct modulation at low currents, and low-cost manufacturing. To keep up with the demand of increasing optical interconnect capacity, the Photonics Laboratory at Chalmers university of Technology is conducting research into improving the speed, reach and capacity of VCSELs for optical interconnects.
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| 3. |
- Karout, Johnny, 1985, et al.
(författare)
-
Designing Power-Efficient Modulation Formats for Noncoherent Optical Systems
- 2011
-
Ingår i: GLOBECOM - IEEE Global Telecommunications Conference. - 9781424492688
-
Konferensbidrag (refereegranskat)abstract
- We optimize modulation formats for the additive white Gaussian noise channel with a nonnegative input constraint, also known as the intensity-modulated direct detection channel, with and without confining them to a lattice structure. Our optimization criteria are the average electrical and optical power. The nonnegativity input signal constraint is translated into a conical constraint in signal space, and modulation formats are designed by sphere packing inside this cone. Some remarkably dense packings are found, which yield more power-efficient modulation formats than previously known. For example, at a spectral efficiency of 1 bit/s/Hz, the obtained modulation format offers a 0.86 dB average electrical power gain and 0.43 dB average optical power gain over the previously best known modulation formats to achieve a symbol error rate of 10^-6. This modulation turns out to have a lattice-based structure. At a spectral efficiency of 3/2 bits/s/Hz and to achieve a symbol error rate of 10^-6, the modulation format obtained for optimizing the average electrical power offers a 0.58 dB average electrical power gain over the best lattice-based modulation and 2.55 dB gain over the best previously known format. However, the modulation format optimized for average optical power offers a 0.46 dB average optical power gain over the best lattice-based modulation and 1.35 dB gain over the best previously known format.
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| 4. |
- Karout, Johnny, 1985, et al.
(författare)
-
Modulation scheme
- 2010
-
Annan publikation (övrigt vetenskapligt/konstnärligt)
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| 5. |
- Karout, Johnny, 1985, et al.
(författare)
-
Optimizing Constellations for Single-Subcarrier Intensity-Modulated Optical Systems
- 2012
-
Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 58:7, s. 4645-4659
-
Tidskriftsartikel (refereegranskat)abstract
- We optimize modulation formats for the additive white Gaussian noise channel with nonnegative input, also known as the intensity-modulated direct-detection channel, with and without confining them to a lattice structure. Our optimization criteria are the average electrical, average optical, and peak power. The nonnegative constraint on the input to the channel is translated into a conical constraint in signal space, and modulation formats are designed by sphere packing inside this cone. Some dense packings are found, which yield more power-efficient modulation formats than previously known. For example, at a spectral efficiency of 1.5 bit/s/Hz, the modulation format optimized for average electrical power has a 2.55 dB average electrical power gain over the best known format to achieve a symbol error rate of 10-6 . The corresponding gains for formats optimized for average and peak optical power are 1.35 and 1.72 dB, respectively. Using modulation formats optimized for peak power in average-power limited systems results in a smaller power penalty than when using formats optimized for average power in peak-power limited systems. We also evaluate the modulation formats in terms of their mutual information to predict their performance in the presence of capacity-achieving error-correcting codes, and finally show numerically and analytically that the optimal modulation formats for reliable transmission in the wideband regime have only one nonzero point.
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| 6. |
- Larsson, Anders, 1957, et al.
(författare)
-
High speed VCSELs for optical interconnects
- 2012
-
Ingår i: Conference Proceedings - International Conference on Indium Phosphide and Related Materials. - 1092-8669. - 9781467317252 ; , s. 269-272
-
Konferensbidrag (refereegranskat)abstract
- This paper presents an overview of our recent work on high speed, oxide confined, 850 nm vertical cavity surface emitting lasers (VCSELs). With proper active region and cavity designs, and techniques for reducing capacitance and thermal impedance, we have reached a modulation bandwidth of 23 GHz and demonstrated 40 Gbps transmission. Using an integrated mode filter for reducing the spectral width we have extended the reach on multimode fiber at 25 Gbps from 100 to 500 m. Improved link capacity was also demonstrated using a more spectrally efficient multi-level modulation format (4-PAM). Finally, a MEMS-technology for wafer scale integration of tunable high speed VCSELs was developed, enabling a tuning range of 24 nm, a 6 GHz modulation bandwidth, and 5 Gbps transmission.
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| 7. |
- Larsson, Anders, 1957, et al.
(författare)
-
VCSEL design and integration for high-capacity optical interconnects
- 2017
-
Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9781510606593 ; 10109, s. 101090M-
-
Konferensbidrag (refereegranskat)abstract
- Vertical-cavity surface-emitting lasers and multi-mode fibers is the dominating technology for short-reach optical interconnects in datacenters and high performance computing systems at current serial rates of up to 25-28 Gbit/s. This is likely to continue at 50-56 Gbit/s. The technology shows potential for 100 Gbit/s.
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| 8. |
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| 9. |
- Safaisini, Rashid, 1981, et al.
(författare)
-
22 Gb/s error-free data transmission beyond 1 km of multi-mode fiber using 850 nm VCSELs
- 2013
-
Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9780819494085 ; 8639
-
Konferensbidrag (refereegranskat)abstract
- The first error-free data transmission beyond 1 km of multi-mode fiber at bit-rates exceeding 20 Gb/s is demonstrated using a high modulation bandwidth, quasi-single mode (SMSR similar to 20 dB) 850 nm VCSEL. A VCSEL with small similar to 3 mu m aperture shows quasi-single mode operation with a narrow spectral width. The top mirror reflectivity of the VCSEL is optimized for high speed and high output power by shallow etching. A combination of narrow spectral width and high optical power reduces the effects of fiber dispersion and fiber and connector losses and enables such a long transmission distance at high bit-rates.
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| 10. |
- Safaisini, Rashid, 1981, et al.
(författare)
-
High-Speed 850 nm Quasi-Single Mode VCSELs for Extended Reach Optical Interconnects
- 2013
-
Ingår i: Journal of Optical Communications and Networking. - 1943-0620 .- 1943-0639. ; 5:7, s. 686-695
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents recent results on high-speed, quasi-single-mode, 850 nm vertical-cavity surface-emitting lasers (VCSELs) with a narrow spectral width for extended-reach optical interconnects. The top mirror reflectivity is adjusted for high output power, slope efficiency, and small signal modulation bandwidth. An oxide confined VCSEL with an ∼3 μm aperture diameter delivers 2 mW of output power and reaches a resonance frequency as high as 25 GHz and a modulation bandwidth exceeding 20 GHz. A small K-factor of 0.17 ns and a large D-factor of 17.3 GHz/mA1/2, extracted from the VCSEL modulation response, along with the improved DC and modal properties enable energy-efficient data transmission at high bit rates over long-distance multimode fiber. Error-free transmission at bit rates exceeding 20 Gbits/s over 1.1 km of OM4 fiber is demonstrated and shown to be limited mainly by the photoreceiver bandwidth. A theoretical investigation of the dependence of link performance on photoreceiver bandwidth is also presented.
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| 11. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
30 Gbps 4-PAM transmission over 200m of MMF using an 850 nm VCSEL
- 2011
-
Ingår i: Optics Express. - 1094-4087 .- 1094-4087. ; 19:26, s. B203-B208
-
Tidskriftsartikel (refereegranskat)abstract
- We present high speed real time, error free 4-PAMtransmissionfor short range optical links based on a VCSEL operating at 850 nm, a multimode fibre and a simple intensity detector. Transmission speeds of 25 Gbps and 30 Gbps are demonstrated, and the maximum fibre reaches were 300 m and 200 m, respectively. The 4-PAM is also compared with OOK transmission at 25 Gbps, and we find that at this bit rate 4-PAM increases the error free transmission distance in the multimode fibre by 100 m, compared to OOK.
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| 12. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
30 Gbps 4-PAM transmission over 200m of MMF using an 850 nm VCSEL
- 2011
-
Ingår i: European Conference and Exposition on Optical Communications, ECOC. - 2162-2701. - 9781557529312 ; , s. 1-3
-
Konferensbidrag (refereegranskat)abstract
- Real time, error free 4-PAM transmission at 25 Gbps and 30 Gbps over 300 m and200m of multimode fiber, respectively, using a VCSEL operating at the wavelength of 850 nm.
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| 13. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
4-PAM for high-speed short-range optical communications
- 2012
-
Ingår i: Journal of Optical Communications and Networking. - 1943-0620 .- 1943-0639. ; 4:11, s. 885-894
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, we compare 4-pulse amplitudemodulation and on–off keying modulation formats at highspeed for short-range optical communication systems. Thetransmission system comprised a directly modulated verticalcavitysurface-emitting laser operating at a wavelength of850 nm, an OM3Å multimode fiber link, and a photodetectordetecting the intensity at the receiver end. The modulationformats were compared both at the same bit-rate and at thesame symbol rate. The maximum bit-rate used was 25 Gbps.Propagation distances up to 600 m were investigated at12.5 Gbps. All measurements were done in real time andwithout any equalization.
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| 14. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
Demonstration of 8-level subcarrier modulation sensitivity improvement in an IM/DD system
- 2011
-
Ingår i: European Conference and Exposition on Optical Communications, ECOC. - Washington, D.C. : OSA. - 2162-2701. - 9781557529312 - 9781557529312
-
Konferensbidrag (refereegranskat)abstract
- We experimentally demonstrate up to 2 dB sensitivity improvement for 8-levelsubcarrier modulation compared to conventional 8-level subcarrier modulation.
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| 15. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
Experimental comparison of modulation formats in IM/DD links
- 2011
-
Ingår i: Optics Express. - 1094-4087 .- 1094-4087. ; 19:10, s. 9881-9889
-
Tidskriftsartikel (refereegranskat)abstract
- We present an experimental comparison of modulation formatsfor optical intensity modulated links with direct detection. Specifically, wecompare OOK, QPSK on an electrical subcarrier and a new modulationformat named OOPSK. The OOPSK modulation format is shown to havebetter sensitivity than the other modulation formats, in agreement withtheoretical predictions. The impact of propagation in multimode fiber isalso studied and the results show that all modulation formats have similarsensitivity penalties, with respect to the fibre length.
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| 16. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
Optimized lattice-based 16-level subcarrier modulation for IM/DD systems
- 2012
-
Ingår i: 2012 38th European Conference and Exhibition on Optical Communications, ECOC 2012; Amsterdam; Netherlands; 16 September 2012 through 20 September 2012. - 9781557529503 ; , s. Art. no. 6705904-
-
Konferensbidrag (refereegranskat)abstract
- We present an experimental demonstration of an optimized 16-level lattice-based single-cycle subcarrier modulation for IM/DD systems at 10 Gbps. The new format has 2.5 dB better sensitivity than single-cycle subcarrier 16-QAM and up to 1 dB better sensitivity than baseband 4-PAM at the same bit rate.
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| 17. |
- Castro, Jose Manuel, et al.
(författare)
-
48.7 Gb/s 4-PAM Transmission over 200 m of High Bandwidth MMF using an 850 nm VCSEL
- 2015
-
Ingår i: IEEE Photonics Technology Letters. - 1041-1135 .- 1941-0174. ; 27:17, s. 1799-1801
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate 4-PAM 48.67 Gb/s transmission, using a directly modulated VCSEL, over 50, 100 and 200 m of multimode fiber with partial dispersion compensation properties. The results of our experiments indicate that 200 m reach is achievable with forward error correcting codes with small overhead.
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| 18. |
- Castro, Jose Manuel, et al.
(författare)
-
50 Gb/s 4-PAM over 200 m of High Bandwidth MMF using a 850 nm VCSEL
- 2015
-
Ingår i: Optical Fiber Communication Conference 2015, Los Angeles, California, United States, 22–26 March 2015. - 9781557529374
-
Konferensbidrag (refereegranskat)abstract
- Experiments for transmission of 50Gb/s, 4-PAM using direct modulated VCSEL over 100m and 200m of MMF with partial dispersion compensation properties are presented.
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| 19. |
- Castro, J. M., et al.
(författare)
-
Investigation of 60 Gb/s 4-PAM Using an 850 nm VCSEL and Multimode Fiber
- 2016
-
Ingår i: Journal of Lightwave Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 0733-8724 .- 1558-2213. ; 34:16, s. 3825-3836
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate 4-pulse-amplitude modulation (PAM) at data rates in the range of 50-60 Gb/s using a directly modulated 850 nm VCSEL and multimode fiber links up to 200 m. The channel conditions in which 4-PAM has performance advantages over traditional ON-OFF keying are evaluated followed by experimental results at data rates of 56.7 Gb/s over distances up to 200 m using equalization and FEC, with a post-FEC bit error rate of 10(-12). The reported analysis is applicable to future Ethernet and Fibre Channel applications requiring 50 Gb/s serial data rates per lane over multimode fibers.
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| 20. |
- Chen, Jingjing, 1982, et al.
(författare)
-
An Energy Efficient 56 Gbps PAM-4 VCSEL Transmitter Enabled by a 100 Gbps Driver in 0.25 µm InP DHBT Technology
- 2016
-
Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 34:21, s. 954-4964
-
Tidskriftsartikel (refereegranskat)abstract
- Datacenters demand higher speed VCSEL-based optical interconnects at low power consumption. As a potential technology enabler, this paper presents a wide bandwidth and energy efficient multilevel PAM VCSEL driver implemented in an InP 0.25 μm double heterojunction bipolar transistor (DHBT) technology. The operational bandwidth of the driver is verified and error-free electrical data transmission up to 56 Gbps PAM-2 and 100 Gbps PAM-4 is demonstrated at a driver energy consumption less than 2 pJ/bit. The driver is integrated and tested with an in-house fabricated 850 nm VCSEL. Error-free 56 Gbps PAM-4 optical transmission at a transmitter energy consumption of 3.7 pJ/bit is demonstrated without equalization. This is the highest data rate reported for an integrated PAM-4 modulated VCSEL transmitter, while being the most energy efficient above 40 Gbps operation. Moreover, the VCSEL driver offers a pre-emphasis feature at PAM-2 operation for improved link throughput and receiver sensitivity.
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| 21. |
- Fougstedt, Christoffer, 1990, et al.
(författare)
-
Low-Power Low-Latency BCH Decoders for Energy-Efficient Optical Interconnects
- 2017
-
Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 35:23, s. 5201-5207
-
Tidskriftsartikel (refereegranskat)abstract
- Since energy dissipation and latency in optical interconnects are of utmost concern, such links are often operated without forward error correction. We propose a low-complexity non-iterative 2-error correcting BCH decoder circuit that significantly relaxes the stringent optical modulation amplitude requirements on the transmitter, which help to reduce laser and laser driver energy. We demonstrate, in a 28-nm CMOS process technology, that the introduction of this BCH circuit in an uncoded link leads to energy-per-bit reductions of 25%, even when impact of code rate on receiver energy dissipation is considered.
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| 22. |
- Gustavsson, Johan, 1974, et al.
(författare)
-
High-speed 850-nm VCSELs for 40 Gb/s transmission
- 2010
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We have explored the possibility to extend the data transmission rate for standard 850-nm GaAs-based VCSELs beyond the 10 Gbit/s limit of today's commercially available directly-modulated devices. By sophisticated tailoring of the design for high-speed performance we demonstrate that 10 Gb/s is far from the upper limit. For example, the thermal conductivity of the bottom mirror is improved by the use of binary compounds, and the electrical parasitics are kept at a minimum by incorporating a large diameter double layered oxide aperture in the design. We also show that the intrinsic high speed performance is significantly improved by replacing the traditional GaAs QWs with strained InGaAs QWs in the active region. The best overall performance is achieved for a device with a 9 μm diameter oxide aperture, having in a threshold current of 0.6 mA, a maximum output power of 9 mW, a thermal resistance of 1.9 °C/mW, and a differential resistance of 80 Ω. The measured 3dB bandwidth exceeds 20 GHz, and we experimentally demonstrate that the device is capable of error-free transmission (BER
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| 23. |
|
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| 24. |
- Lengyel, Tamas, 1986, et al.
(författare)
-
Impact of Damping on 50 Gbps 4-PAM Modulation of 25G Class VCSELs
- 2017
-
Ingår i: Optics InfoBase Conference Papers. - 2162-2701. - 9781943580231 ; , s. paper W3G.2-
-
Konferensbidrag (refereegranskat)abstract
- We investigate the effects of photon lifetime and damping of the modulation response on the quality of 50 Gbps 4-PAM signal generation with directly modulated 25G class VCSELs and identify the appropriate values for the K-factor.
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| 25. |
- Lengyel, Tamas, 1986, et al.
(författare)
-
Impact of Damping on 50 Gbps 4-PAM Modulation of 25G Class VCSELs
- 2017
-
Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 35:19, s. 4203-4209
-
Tidskriftsartikel (refereegranskat)abstract
- We present an investigation into the effects of photon lifetime and the associated damping of the modulation response on 50 Gbps 4-PAM signal generation with directly modulated 25G class VCSELs. While 4-PAM benefits from higher slope efficiency and output power, it is shown that the greater impact of intensity noise and dynamic nonlinearities implies that a VCSEL with a longer photon lifetime and more damped response is needed when transitioning from existing 25 Gbps OOK to 50 Gbps 4-PAM using the 25G VCSEL technology.
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| 26. |
- Lengyel, Tamas, 1986, et al.
(författare)
-
Sensitivity Improvements in an 850 nm VCSEL-based Link using a Two-tap Pre-emphasis Electronic Filter
- 2016
-
Ingår i: Journal of Lightwave Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 0733-8724 .- 1558-2213. ; 35:9, s. 1633-1639
-
Tidskriftsartikel (refereegranskat)abstract
- We present an investigation of an ISI penalty reduction for On-Off Keying in an 850 nm VCSEL-based short optical link using a two-tap electronic pre-emphasis filter and quantify the improvement relative to an equivalent link without pre-emphasis. Error-free operation is demonstrated at bit rates above the −3 dB modulation bandwidth of the link. Sensitivity improvement up to 9 dB compared to non-emphasized signaling is shown. A maximum error-free bit rate achievable in the link is increased to 57 Gbps, compared to 52 Gbps without the pre-emphasis.
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| 27. |
- Lengyel, Tamas, 1986, et al.
(författare)
-
Sensitivity improvements in an 850 nm VCSEL transmitter using a one-tap pre-emphasis electronic filter
- 2015
-
Ingår i: 41st European Conference on Optical Communication, ECOC 2015, Valencia, Spain, 27 September - 1 October 2015. - 9788460817413 ; 2015-November
-
Konferensbidrag (refereegranskat)abstract
- We present ISI reduction of an 850 nm VCSEL-based back-to-back optical link using a one-tap electronic pre-emphasis filter and compare it to a VCSEL link without filtering with similar launch conditions. Error floors are eliminated by the use of the filter.
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| 28. |
- Lin, Rui, et al.
(författare)
-
Scalability analysis methodology for passive optical interconnects in data center networks using PAM
- 2017
-
Ingår i: Optics Communications. - : Elsevier. - 0030-4018 .- 1873-0310. ; 403, s. 283-289
-
Tidskriftsartikel (refereegranskat)abstract
- A framework is developed for modeling the fundamental impairments in optical datacenter interconnects, i.e., the power loss and the receiver noises. This framework makes it possible, to analyze the trade-offs between data rates, modulation order, and number of ports that can be supported in optical interconnect architectures, while guaranteeing that the required signal-to-noise ratios are satisfied. To the best of our knowledge, this important assessment methodology is not yet available. As a case study, the trade-offs are investigated for three coupler-based top-of-rack interconnect architectures, which suffer from serious insertion loss. The results show that using single-port transceivers with 10 GHz bandwidth, avalanche photodiode detectors, and quadratical pulse amplitude modulation, more than 500 ports can be supported.
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| 29. |
- Lin, Rui, et al.
(författare)
-
To overcome the scalability limitation of passive optical interconnects in datacentres
- 2016
-
Ingår i: Optics InfoBase Conference Papers. - : OSA - The Optical Society. - 2162-2701. - 9780960038008
-
Konferensbidrag (refereegranskat)abstract
- We propose to add optical amplifier(s) to passive optical interconnect (POI) at top-of-rack in datacentres and validate this approach by introducing impairment constraints into POIs design. It is shown that one amplifier can improve scalability by a factor of 16.
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| 30. |
- Rastegarfar, Houman, 1983, et al.
(författare)
-
Optical Circuit Granularity Impact in TCP-Dominant Hybrid Data Center Networks
- 2017
-
Ingår i: 2017 International Conference on Computing, Networking and Communications, ICNC 2017. - 9781509045884 ; , s. 318-322
-
Konferensbidrag (refereegranskat)abstract
- Hybrid networking, based on electronic packet switching and optical circuit switching, has been proposed to resolve the existing switching bottlenecks in data centers in an energy-efficient and cost-effective fashion. We consider the problem of resource provisioning in hybrid data centers in terms of optical circuit switching capacity and granularity. The number of fibers connected to server racks, the number of wavelengths per fiber, and the ratio of capacity provided by the optical circuit-switched portion of the network to that of the electronic packet-switched portion are crucial design parameters to be optimized during the data center planning phase. These parameters in conjunction with the additive-increase, multiplicative-decrease (AIMD) congestion control mechanism of the Transmission Control Protocol (TCP) pose a significant impact on data center network performance. In this paper, we examine the combined impact of optical bandwidth settings and TCP dynamics using event-driven simulations. Our analysisreveals the strong dependence of overall network throughput on channel capacity (i.e., the bit rate per wavelength channel) and points to the advantages of optical bandwidth consolidation employing higher-order modulation formats.
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| 31. |
- Rastegarfar, Houman, 1983, et al.
(författare)
-
PAM Performance Analysis in Multicast-Enabled Wavelength-Routing Data Centers
- 2017
-
Ingår i: Journal of Lightwave Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 0733-8724 .- 1558-2213. ; 35:13, s. 2569-2579
-
Tidskriftsartikel (refereegranskat)abstract
- Multilevel pulse amplitude modulation (M-PAM) is gaining momentum for high-capacity and power-efficient cloud computing. Compared to the classic on-off keying (OOK) modulation, high-order PAM yields better spectral efficiency but is also more susceptible to physical layer degradation effects. We develop a cross-layer analysis framework to examine the PAM transmission performance in data center network environments supporting both optical multicasting and wavelength routing. Our analysis is conducted on a switch architecture based on an arrayed-waveguide grating (AWG) core and distributed broadcast domains, exhibiting different physical paths, and random, uncontrolled crosstalk noise. Reed-Solomon coding with rate adaptation is incorporated into PAM transceivers to compensate for impairments. Our Monte Carlo simulations point to the significant impact of AWG crosstalk on higher order PAM in wavelength-reuse architectures and the importance of code rate adaptation for signals traversing multiple routing stages. According to our study, 8-PAM offers the highest effective bit rates for signals terminating in one broadcast domain and performs poorly when considering interdomain connectivity. On the other hand, the impairment-induced degradation of interdomain capacity for 4-PAM can be limited to 20.7%, making it better suited for connections spanning two broadcast domains and a crosstalk-rich stage. Our results call for software-defined PAM transceiver designs in support of both modulation order and code rate adaptation.
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| 32. |
- Srinivasan, Muralikrishnan, 1991, et al.
(författare)
-
End-to-End Learning for VCSEL-based Optical Interconnects: State-of-the-Art, Challenges, and Opportunities
- 2023
-
Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 41:11, s. 3261-3277
-
Tidskriftsartikel (refereegranskat)abstract
- Optical interconnects (OIs) based on vertical-cavity surface-emitting lasers (VCSELs) are the main workhorse within data centers, supercomputers, and even vehicles, providing low-cost, high-rate connectivity. VCSELs must operate under extremely harsh and time-varying conditions, thus requiring adaptive and flexible designs of the communication chain. Such designs can be built based on mathematical models (model-based design) or learned from data (machine learning (ML) based design). Various ML techniques have recently come to the forefront, replacing individual components in the transmitters and receivers with deep neural networks. Beyond such component-wise learning, end-to-end (E2E) autoencoder approaches can reach the ultimate performance through co-optimizing entire parameterized transmitters and receivers. This tutorial paper aims to provide an overview of ML for VCSEL-based OIs, with a focus on E2E approaches, dealing specifically with the unique challenges facing VCSELs, such as the wide temperature variations and complex models.
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| 33. |
- Srinivasan, Muralikrishnan, 1991, et al.
(författare)
-
Learning Optimal PAM Levels for VCSEL-based Optical Interconnects
- 2022
-
Ingår i: 2022 European Conference on Optical Communication, ECOC 2022. - 9781957171159
-
Konferensbidrag (refereegranskat)abstract
- An auto-encoder that optimizes a VCSEL-based fiber-optic system end-to-end and provides a 1.5dB sensitivity gain at higher temperatures is trained, utilizing a neural network that models the response of a VCSEL for a range of operating temperatures.
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| 34. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
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35.2 Gbps 8-PAM transmission over 100 m of MMF using an 850 nm VCSEL
- 2013
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Ingår i: Proc. European Conference on Optical Communication (ECOC). - : Institution of Engineering and Technology. - 9781849197595 ; 2013:622 CP, s. 744-746
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Konferensbidrag (refereegranskat)abstract
- We report experimental demonstration of 8-PAM transmission using an 850 nm VCSEL and 100 m of OM4-type MMF. The 8-level driving signal was generated using a 3-bit DAC, the error rates were measured in real time using a conventional error analyzer. Maximum uncoded bit rate was 37.5 Gbps, which corresponds to 35.2 Gbps with 7% FEC overhead.
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| 35. |
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| 36. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
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60 Gbits error-free 4-PAM operation with 850 nm VCSEL
- 2013
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Ingår i: Electronics Letters. - : Institution of Engineering and Technology (IET). - 1350-911X .- 0013-5194. ; 49:15, s. 953-955
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Tidskriftsartikel (refereegranskat)abstract
- 60 Gbits over 2 m, 50 Gbits over 50 m and 40 Gbits was transmitted over 100 m of OM4 multimode fibre using four-level pulse amplitude modulation and a directly modulated 850 nm vertical cavity surface emitting laser (VCSEL).
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| 37. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
70 Gbps 4-PAM and 56 Gbps 8-PAM Using an 850 nm VCSEL
- 2015
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Ingår i: Journal of Lightwave Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 0733-8724 .- 1558-2213. ; 33:7, s. 1395-1401
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Tidskriftsartikel (refereegranskat)abstract
- We present 56 Gbps unequalized 8-pulse-amplitude-modulation (PAM) real-time transmission over 50 m of multimode fiber and 70 Gbps 4-PAM operation with offline equalization. FEC would be necessary in both schemes. The bit-rates are given taking into account the FEC overhead needed to reach a BER below 10-12. The experiments were performed with an 850 nm vertical cavity surface emitting laser with 20 GHz bandwidth and a 22 GHz photoreceiver. We show that 8-PAM modulation requires channel bandwidth comparable with the symbol rate to avoid excessive ISI penalty and that equalization of 4-PAM signal can significantly improve achievable bit-rates.
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| 38. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
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70 Gbps 4-PAM and 56 Gbps 8-PAM using an 850 nm VCSEL
- 2014
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Ingår i: 2014 European Conference on Optical Communication, ECOC 2014; Cannes; France; 21 September 2014 through 25 September 2014. - 9782954944401 ; , s. Art. no. 6963923-
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Konferensbidrag (refereegranskat)abstract
- t We present 56 Gbps unequalized 8-PAM real-time transmission over 50 m of MMF and 70Gbps 4-PAM operation with offline equalization. The experiments were performed with an 850 nm VCSEL with 25 GHz bandwidth and a 22 GHz photoreceiver.
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| 39. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
94-Gb/s 4-PAM Using an 850-nm VCSEL, Pre-Emphasis, and Receiver Equalization
- 2016
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Ingår i: IEEE Photonics Technology Letters. - 1041-1135 .- 1941-0174. ; 28:22, s. 2519-2521
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Tidskriftsartikel (refereegranskat)abstract
- We present the results from an experimental demonstration of a 100-Gb/s 4-PAM transmission over a few meter MMFs using an 850-nm VCSEL with both analog pre-emphasis and a digital offline equalizer. The effective data rate is 94 Gb/s after accounting for the forward error correction overhead. Digital equalizers of lengths 11 to 71 taps were investigated.
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| 40. |
- Szczerba, Krzysztof, 1985
(författare)
-
Advanced modulation formats for short-range optical communications
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The demand for faster short range optical links is increasing and simple modulation techniques, such as on-o keying do not scale well to higher data rates, because of the limited bandwidth-distance product of the multimode ber. The industry has responded to this problem by aggregating multiple parallel links operating at lower bit-rate. Another possible way of improving the short range optical interconnect is to used better, more advanced modulation formats.Advanced modulation formats in the context of short range optical communication are the topic of this thesis. A broad spectrum of modulation formats have been studied, starting with pulse amplitude modulation, through conventional subcarrier modulation, to a new class of three-dimensional subcarrier modulation. Although these modulation formats are very diverse, they also have one aspect in common; theycan be implemented in systems using intensity modulation and direct detection. The modulation formats were investigated experimentally in an optical link using a directly modulated vertical cavity surface emitting laser as a transmitter, photodiode for directdetection in the receiver and multimode ber as the transmission medium.The relatively simple pulse amplitude modulation was the easiest one to implementat high bit-rates; 30 Gbps error free real time transmission was demonstrated. The subacarrier modulation is more complex and required o-line signal processing, however operation at bit-rates as high as 37 Gbps were demonstrated. Finally, modulation formats for IM/DD links with improved sensitivity were investigated, showing 0.6 dB better sensitivity and the same spectral efficiency as OOK. More complex format with 8 levels and 2 dB better sensitivity than subcarrier 8-QAM was also demonstrated.
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| 41. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
Comparison of Intersymbol Interference Power Penalties for OOK and 4-PAM in Short-Range Optical Links
- 2013
-
Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 31:22, s. 3525-3534
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Tidskriftsartikel (refereegranskat)abstract
- We present results of experimental and theoretical investigations of intersymbol interference in 4-PAM transmission in short-range optical communications links based on the power penalty. A test link comprised of a directly modulated 850 nm VCSEL with up to 200 m of multimode fiber and direct detection was used. The link bandwidth was below 10 GHz and the maximum achieved data rate with 4-PAM was 44 Gbps over 100 m of fiber. In the same case and at similar sensitivity, only 32 Gbps could be achieved with OOK. If typical forward error correction could be applied, the sensitivity of the 4-PAM system was improved by up to 4 dB, reaching -10 dBm at 25 Gbps.
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| 42. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
Energy Efficiency of VCSELs in the Context of Short-Range Optical Links.
- 2015
-
Ingår i: IEEE Photonics Technology Letters. - 1041-1135 .- 1941-0174. ; 27:16, s. 1749-1752
-
Tidskriftsartikel (refereegranskat)abstract
- We present results from an investigation of theenergy efficiency of VCSELs under large signal modulation. We show that the most important factor influencing the energy consumption of the VCSELs is the required optical modulation amplitude, which drives other VCSEL design requirements. The required optical modulation amplitude also depends on the optical link design. Through this dependence it is possible to better understand the energy consumption of complete short range optical links.
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| 43. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
High speed optical interconnects with 850 nm VCSELs and advanced modulation formats
- 2017
-
Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 10122
-
Konferensbidrag (refereegranskat)abstract
- We present our recent work on high-speed optical interconnects with advanced modulation formats and directly modulated 850 nm VCSELs. Data transmission at nearly 100 Gbps was achieved with 4-PAM. Forward error correction, equalization and preemphasis are also explored. The system aspects of the advanced modulation formats and their impact on the VCSEL requirements are discussed. Requirements on the optical output power, frequency response and the relative intensity noise are discussed. Finally, co-optimization of the VCSELs and VCSEL driver amplifiers in CMOS and InP technologies is discussed. © 2017 SPIE.
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| 44. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
Impact of Forward Error Correction on Energy Consumption of VCSEL-based Transmitters
- 2015
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Ingår i: 41st European Conference on Optical Communication, ECOC 2015, Valencia, Spain, 27 September - 1 October 2015. - 9788460817413 ; 2015-November
-
Konferensbidrag (refereegranskat)abstract
- We present an analysis of the effect of FEC on energy consumption of VCSEL based transmitters for short-range optical links. We show that a low-complexity FEC can reduce the overall energy consumption of the transmitter.
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| 45. |
- Szczerba, Krzysztof, 1985, et al.
(författare)
-
Intersymbol interference penalties for OOK and 4-PAM in short-range optical communications
- 2013
-
Ingår i: 2013 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, OFC/NFOEC 2013. - 9781479904570
-
Konferensbidrag (refereegranskat)abstract
- Intersymbol interference penalties of OOK and 4-PAM were compared in the context of short-range optical links with directly modulated VCSELs. The maximum achieved bit rate with 4-PAM was 38 Gbps and 32 Gbps with OOK.
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| 46. |
- Szczerba, Krzysztof, 1985
(författare)
-
Multilevel modulation in short-range optical links
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Power and cost efficient short-range optical links are one of the technologies enabling large scale high-performance computers and data centers. Such links typically use directly modulated vertical cavity surface emitting lasers, multi-mode fiber, direct detection and on-off keying (OOK) modulation. In this work replacement of OOK with multilevel modulation formats is studied, with possible application in future optical interconnects. Subcarrier modulation and pulse amplitude modulation (PAM) were studied because they can be implemented in systems using intensity modulation and direct detection. Previously known subcarrier modulation formats were found to use the optical power inefficiently. In this thesis optimized subcarrier formats were demonstrated experimentally, in particular a three-dimensional four-level optimized subcarrier format called on-off phase-shift keying with 0.6~dB sensitivity improvement over OOK and 2~dB improvement over subcarrier quadrature phase shift-keying at the same bit rate. Similar sensitivity improvements were demonstrated for eight- and sixteen-level subcarrier formats. PAM offers a good trade-off between complexity and sensitivity. It was demonstrated in real-time at bit-rates of up to 60~Gbps with four levels and up to 35~Gbps with eight levels. Practical aspects of its implementation, such robustness to intersymbol interference and application of forward error correction (FEC) were investigated theoretically and experimentally. It was shown that 4-PAM with FEC can double the throughput relative to OOK in the same system, while retaining similar sensitivity.
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