Link Bandwidth and Transmission Capability of Single-Mode Multi-Aperture Vertical-Cavity Surface-Emitting Lasers at 100 G/Lane and 200 G/Lane over Multimode Fibers

Abstract

Single-mode (SM) vertical-cavity surface-emitting lasers (VCSELs) have often been demonstrated with an unusually long transmission reach at very high data rates while today’s multimode VCSEL transmission has been limited by the fiber modal bandwidth and bandwidth contributed by the VCSEL–chromatic dispersion interaction under typical encircled flux launch condition. By using the same launch condition for VCSEL and modal bandwidth measurements, we studied the link bandwidth capability of SM multi-aperture (MA) VCSEL transmission. Using a multimode fiber with modal bandwidth under actual launch conditions moderately lower than OM4 threshold, we observed that the link bandwidth, with contributions from both modal bandwidth and laser–chromatic dispersion interaction, is higher than the corresponding modal bandwidths, which is very counter-intuitive. A detailed analysis reveals that the enhanced link bandwidth is contributed by both narrow laser linewidth and favorable laser–chromatic dispersion interaction. Through the study, we demonstrate that OM4 can meet link bandwidth requirements for 200/100 G/lane transmission over 100/200 m using SM MA VCSELs.

1. Introduction

Vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers (MMFs) have been extensively deployed for short-reach communications [1,2] with the distance spanning from a few meters up to 100 m or more. The rise in AI and GPU-based computing has indeed catalyzed this acceleration in data rate increases, and the latest GPU architectures are now paving the way towards 200 G speeds [3].

As the data rate moves to 100 G/lane, standardized by IEEE 802.3 db [4], the transmission reach limitation becomes more severe. Additionally, 200 G/lane transmission is the next major milestone for VCSEL-MMF transmission [5,6,7], and the current expectation is to transmit over 30–50 m over OM4. However, even transmitting over 50 m of OM4 is considered challenging.

One way to increase the transmission distance of the MMF-based link is to reduce the spectral width of the signal through the use of single-mode (SM) VCSELs [8,9]. Small-aperture SM VCSELs have a narrow spectral width but suffer from high resistance and low output power. To improve SM VCSEL performance, multi-aperture VCSEL arrays have been proposed to increase the output power and reduce resistance [10]. With such devices, data transmission up to 800 m of MMF at 56 Gbaud has been demonstrated [11,12].

High-data-rate SM VCSEL transmission has been demonstrated over several hundred meters to kilometers of MMF at 25–50 Gbaud [11,12,13,14,15], while the exact modal bandwidth of the MMFs used were not specified. The actual deployment of MMFs in the field is based on standard defined grades, such as OM3 and OM4. OM4 has a specified “worst case” effective modal bandwidth (EMB) threshold of 4700 MHz·km at 850 nm. A long-reach demonstration can either use an OM4 fiber with very high EMB or adopt a novel launch condition, for example, exciting only the fundamental mode of the MMF [15,16,17]. Even for an OM3 or OM4 with a nominal EMB close to its threshold, an individual launch condition can still exhibit much higher modal bandwidth. The situation makes it difficult to judge what the true transmission reach capability is for a particular technology. While hero experiments are good for scientific demonstrations, a real industry solution is based on the transmission reach capability for worst-case situations or the lowest EMB defined for a product. On the other hand, SM VCSEL is a VCSEL having a small aperture. It can achieve high bandwidth at low current levels but suffers from high resistance (>150 Ohms), which is incompatible with modern drivers. Additionally, even at current densities sufficient for high modulation bandwidths, the SM device may still have inadequate power. Over-pumping does not offer any benefits due to thermal roll-over and can negatively impact reliability. In contrast, an MA VCSEL reduces the effective resistance to 40–60 Ohms while maintaining small aperture diameters, eliminating the need for over-pumping. Furthermore, the smaller the aperture, the longer the VCSEL’s lifetime is at the same current density due to significantly reduced junction temperature.

The goal of the current work is to study the SM multi-aperture (MA) VCSEL transmission over MMF around OM4 level to determine the transmission capability for all OM4, not limited to those with very high EMB. We built both an SM MA VCSEL setup and a traditional modal bandwidth measurement setup that have essentially the same launch condition. We also chose a set of MMFs that have modal bandwidth measured at encircled flux launch conditions moderately lower than the 4700 MHz·km threshold and another set of MMFs with modal bandwidth mostly above the 4700 MHz·km threshold. Our results demonstrate that, using the SM MA VCSEL with standard launch conditions, all OM4 can meet the link bandwidth requirements for 100/200 G/lane transmission over 200/100 m for practical data center applications.

2. VCSEL Design

The SM MA VCSEL consists of four 850 nm oxide-confined VCSELs as depicted in Figure 1. The four small apertures are located ~10–12 µm apart in a rectangular pattern. Such design results in SM MA VCSEL with a root mean squared (RMS) spectral linewidth of ~0.1 nm, while having output power and resistance values equivalent to MM VCSELs currently used in transceivers. The device is polarization stable with S₂₁ of 27 GHz at room temperature and 25 GHz at 85 °C, both with 8 mA bias current. The total surface area of 4 apertures is approximately equivalent to a single circular aperture having a 6 µm diameter. Each aperture acts as an independent VCSEL at its own wavelength even though the integrated spectrum has low RMS. More information on the epitaxial design and performance of MA VCSEL can be found in Ref. [10]. Relative intensity noise is lower than −152 dB/Hz. Figure 2 shows fiber-coupled output power, voltage as a function of driving current, and optical spectra of the studied devices at room temperature.

3. Experimental Setup and Launch Condition

Figure 3 shows the experimental setup with SM MA VCSEL. We coupled the VCSEL light into the MMF with the FC/PC connector through an aspheric lens with ~1.5 mm working distance and NA of 0.3. At 7 mA bias current, we observed 4.9 dBm optical power coupled into the MMF, which is about 65% of optical power emitted from the VCSEL. The optical power is adequate for data transmission at 100 G and 200 G. The photodetector is a module from the VI-System with 30 GHz bandwidth. The system level S₂₁ parameter and the MMF link level S₂₁ parameter with back-to-back (BtB) system calibrated out were measured through the VNA (PNA-L N5230C, Agilent, Beijing, China). In this paper, the term S₂₁ parameter refers to the magnitude of the S₂₁ parameters, |S₂₁|. Using SM MA VCSEL, the S₂₁ measured reveals the link bandwidth carrying both contributions from modal bandwidth and from laser–chromatic dispersion (CD) interaction.

Figure 4 shows the second experimental setup. It utilizes a narrow linewidth 850 nm light source (BS-840, Superlum, Cork, Ireland) with a linewidth around 0.05 nm. The light is modulated by an intensity modulator and the light launched into MMF is conditioned by a modal conditioner from Ardent ModCon to have an encircled flux (EF) launch condition. Because of the narrow linewidth of the light source, the CD-contributed bandwidth is negligible. In this setup, the S₂₁ is contributed primarily by the modal dispersion. Therefore, we can obtain the modal bandwidth using this measurement configuration.

We also measured the encircled flux from both setups using an EF measurement instrument (Ardent MPX-1, CERN, Geneva, Switzerland), as shown in Figure 5. The launch conditions from both setups are essentially the same and meet the standard requirements for VCSEL transmission. By having the same launch condition from both setups with one measuring link bandwidth and another one measuring modal bandwidth, we can determine the contribution from laser–CD interaction, which helps to determine the transmission capability.

4. Measurement Results of Link Bandwidth and Modal Bandwidth

We prepared two sets of MMFs based on two MMFs. The first set of MMFs, labeled as ‘MMF1’, has modal bandwidth for the launch condition moderately below the OM4 threshold of 4700 MHz·km, while the second set of MMFs, labeled ‘MMF2’, has modal bandwidth at the specific launch condition moderately above the OM4 threshold for most cases. For both sets, the MMFs were prepared in 100/200/300/400 m lengths, respectively. Using the SM MA VCSEL system in Figure 3, the S₂₁ of the system with ‘MMF1’ are shown in Figure 6a. Oscillations in the modulation response may be caused by the optical feedback effect due to optical resonators formed by the coupling optics. With an increased MMF length, the S₂₁ reaches the −3 dBe level at a lower frequency. The S₂₁ contributed from the fiber link after calibrating out the BtB system contribution is shown in Figure 6b, from which we can extract the −3 dBe fiber link bandwidth with both CD and modal dispersion contributions. With up to 200 m of ‘MMF1’, the −3 dBe link bandwidth stays at 22 GHz. At 100 m in length, the −3 dBe link bandwidth is well above 40 GHz, the upper limit of the VNA frequency range. The S₂₁ contributing the modal bandwidth from the same set of MMFs is measured using the setup in Figure 4 and shown in Figure 7. The modal bandwidth, which is optical bandwidth, can be extracted from the $-6\mathrm{d}\mathrm{B}\mathrm{e}(=-3\mathrm{d}\mathrm{B}\mathrm{o})$ level. Note that dBe is defined based on $20·log\left(\right)$ operator while dBo is defined based on $10·log\left(\right)$ operator. It is important to note that those working on transmission systems or in the electrical domain often use a −3 dBe threshold to define bandwidth, such as the case in Ref. [5]. Conversely, in the l field of optics, the modal bandwidth of MMF is typically specified using a −3 dBo or −6 dBe threshold. In our work, we adhere to the conventions used in the respective fields. The extracted link and modal bandwidths are summarized in

Table 1. Extracted modal bandwidths and link bandwidths for ‘MMF1’.

Fiber Length (m)	Modal BW (GHz)	Scaled Modal BW (GHz·km)	Link BW at −3 dBe (GHz)	Link BW at −6 dBe (GHz)
100	38.3	3.83	≫40	≫54.8
200	21.2	4.25	22.5	31.4
300	13.7	4.12	16.4	22.3
400	11.3	4.50	10.8	16.3

. In the table, ‘BW’ is an abbreviation of bandwidth.

The measurements were also conducted for ‘MMF2’. Using the setup in Figure 3, we obtained the S₂₁ of the system in Figure 8a while the S₂₁ contributed by the MMF link is shown in Figure 8b. Using the setup in Figure 4, we obtained the S₂₁ of the fiber in Figure 9 for extracting the modal bandwidth of the fiber. The extracted link and modal bandwidths are summarized in

Table 2. Extracted modal bandwidths and link bandwidths for ‘MMF2’.

Fiber Length (m)	Modal BW (GHz)	Scaled Modal BW (GHz·km)	Link BW at −3 dBe (GHz)	Link BW at −6 dBe (GHz)
100	45.0	4.50	≫40	≫54.8
200	25.0	4.99	24.5	34.6
300	17.8	5.34	16.8	23.7
400	13.5	5.39	12.8	18.0

. With up to 200 m ‘MMF2’, the −3 dBe link bandwidth stays at 24.6 GHz. At 100 m length, the −3 dBe link bandwidth is well above 40 GHz.

It should be noted that previous studies have shown that coupling between VCSEL and MMF can introduce radial spectral dependence over the MMF [18]. This spectral dependence can impact transmission performance, as it interacts with different spatial modes in an MMF, which can compensate or enhance the chromatic dispersion effect depending on fiber DMD characteristics. The root cause of this effect is the presence of multiple modes in multimode VCSELs (MM VCSELs), with each mode associated with a slightly different wavelength. However, in the current study, we investigate SM MA VCSEL transmission. Since the VCSEL has only one mode, the coupling from the SM MA VCSEL to the MMF is free from spectral dependence. Consequently, the coupling-related performance variability expected for MM VCSELs does not occur with SM MA VCSEL transmission.

5. Analysis of Mechanism and Transmission Capability

Since the launch conditions used to obtain link bandwidths and modal bandwidths are essentially the same, we can better understand the laser–CD interaction. Also, since the modal bandwidths of four fibers based on ‘MMF1’ are moderately lower than the EMB threshold of OM4, they set a low-end boundary and OM4 would perform better. We observed that, for all cases, the link bandwidths as set at −6 dBe level are better than the modal bandwidth, which is very counter-intuitive as the laser–CD interaction usually results in worse bandwidth and transmission penalty [2]. We attribute the results to two physical effects:

• Narrow linewidth of SM MA VCSEL [8];
• Favorable laser chirp–CD interaction [19].

Table 3. −3 dBe link bandwidths for OM4 at 850 nm.

Fiber Length (m)	BW (GHz)	Link BW at 0.1 nm Linewidth (GHz)	Link BW at 0.6 nm Linewidth (GHz)
100	47.00	33.3	19.16
200	23.50	16.7	9.58
300	15.65	11.1	6.39
400	11.75	8.3	4.79

shows modeled link bandwidth for different linewidth assumptions using OM4. The modeling has used the CD value defined by the standard [20] following the procedure in Refs. [2,21] to calculate the bandwidth contributed from laser–CD interaction and the link bandwidth. For the MM VCSELs, with the linewidth set at 0.6 nm, the required link bandwidth is established at 18 GHz at the −3 dBe level for 100 m transmission at 100 G/lane [5]. In this study, the transmission capability is determined based on meeting this link bandwidth requirement. For 100 G/lane, we adhere to the 18 GHz level to ensure valid transmission. For 200 G/lane, we anticipate needing more than 2 times of link bandwidth for 100 G/lane to achieve the same transmission reach. It is well known that a narrower laser linewidth results in lower CD-contributed bandwidth. With a narrower linewidth of 0.1 nm, the transmission is capable of going 180 m, but still not enough to reach 200 m. In addition, the link bandwidth is always lower than the modal bandwidth even when the laser linewidth is 0.1 nm. This means narrow linewidth alone cannot explain the experimental results presented in Table 1 and Table 2.

The second mechanism is the favorable interaction between laser chirp and fiber CD [19]. An SM VCSEL from VI-Systems was reported to have significant chirp ($\alpha ~-3.52$) that results in transmission benefit with negative CD in MMF. The CD of the MMF at 850 nm is around −100 ps/(nm·km) so the interaction can have a significant effect even for fibers at very short lengths of less than 200 m.

The favorable chirp–CD interaction can explain the additional link bandwidth improvement beyond what is allowed by narrow linewidth as shown in Table 3. For ‘MMF 1’, at 200 m, the link bandwidth is well above the IEEE definition for 100 G/lane transmission. As a rule of thumb, the link bandwidth required for 200 G/lane would be at least 2 times of what is needed at 100 G/lane. The ≫40 GHz value for 100 m MMF in Table 1 would likely meet the 200 G/lane requirements.

The four fibers based on ‘MMF2’ have modal bandwidths higher than ‘MMF1’, mostly above the OM4 threshold. The link bandwidths of ‘MMF2’ are also moderately better than ‘MMF1’. With such incremental improvements on the modal bandwidth, overall link bandwidths at four lengths are also improved and meet the 100 G and 200 G transmission requirements at 200 m and 100 m, respectively.

6. Transmission Result over 200 m MMF

We obtained 50 Gbaud NRZ eye diagrams for two of the fibers of interest, i.e., 100 m and 200 m of ‘MMF1’. This allows us to visualize the transmission capability under large amplitude modulation. The VCSEL was directly driven by an SHF bit pattern generator with a digital signal without equalization. The signal was acquired with Tektronix DSA with the integrated optical receiver. Figure 10 shows the bit error contours for transmission over 100 m and 200 m of ‘MMF1’ showing error-free transmission. Since this MMF has modal bandwidth at the launched condition used for the experiment below OM4 threshold, the eye diagram supports the results from the link bandwidth measurement indicating all OM4, which has higher modal bandwidth, can transmit at this length.

7. Discussion

In this work, we studied the link bandwidth and transmission capability based on using SM MA VCSEL and analyzed the transmission mechanism. Below, we compare several contributing factors affecting the transmission capability and highlight the factors critical in the current study for supporting high-data-rate VCSEL transmission.

• Modal bandwidth: The quality of an MMF is characterized by its modal bandwidth or EMB at the operating wavelength. As defined by the standard, the highest grade of MMF is called OM4, which has 4700 MHz·km EMB at 850 nm. It is often desirable to have a higher EMB to support a higher data rate and longer transmission reach. At the link level, the link bandwidth, which has the combined effect of modal bandwidth and bandwidth from laser–CD, is what matters to the transmission. Specifically, link bandwidth is related to modal bandwidth, ${BW}_{modal}$, and bandwidth from the laser–CD interaction, ${BW}_{CD}$ using the following equation [21],

  $${BW}_{Link}=({BW}_{modal}^{-2}+{BW}_{CD}^{-2}{)}^{-1/2}$$

  (1)
  However, when the laser linewidth is high, for example, at 0.6 nm used in the current standard [5], the laser–CD interaction contributes 3.2 GHz·km of bandwidth. Therefore, increasing EMB is helpful but is ultimately limited by the bandwidth from laser–CD interaction if laser linewidth stays high.
• Laser linewidth and laser power: It is well known that reduced VCSEL linewidth can lead to much higher link bandwidth [8,9]. Table 3 illustrates the results of 0.1 nm and 0.6 nm. When SM VCSEL with narrow linewidth is used, the link bandwidth is much higher for the same modal bandwidth when compared to a laser linewidth of 0.6 nm. Another factor is the SM VCSEL can suffer from high current density, which may limit the reliability of the VCSEL. The use of SM MA VCSEL can alleviate such issue by having multiple SM VCSELs on one device, with each contributing lower optical power [10].
• Favorable laser–CD interaction: SM VCSEL can have a favorable interaction between laser chirp and fiber CD [19]. The SM VCSEL from VI-Systems has been reported to have significant chirp (α ~ −3.52) that results in transmission benefit with negative CD in optical fibers. The CD of the MMF at 850 nm is around −100 ps/(nm·km) so the interaction can have a significant effect even at very short lengths as short as 100–200 m.

The current work identifies the mechanisms of unusual transmission capability exhibited by SM MA VCSEL, which emphasizes the importance of using narrow laser linewidth associated with SM VCSEL and strong laser chirp of VCSEL working with highly negative CD of MMF around 850 nm. Such study can be extended to longer wavelength, such as 940 nm, 980 nm, 1060 nm or even above 1100 nm, where the fiber CD values are lower. We note that a favorable effect of laser chirp on data transmission was also used to address the transmission for single mode fiber WDM transmission, with significantly improves in BER by as much as 8 orders of magnitude at the short-wavelength boundary of the O-Band over the long wavelength boundary [22]. The favorable chirp interaction with the CD of transmission fibers can have broader implications than what is studied in the current work. For example, long distance data transmission at wavelengths much shorter than 1260 nm may provide further advantages for single-mode transmission.

8. Conclusions

SM MA VCSELs have exhibited unusual transmission capability better than traditional MM VCSELs with higher link bandwidth resulting in longer system reaches. By using the same launch condition in VCSEL and modal bandwidth measurements, we studied the link bandwidth capability of SM MA VCSEL transmission and identified the mechanisms that support the high-data-rate and longer-reach transmissions. Our analysis shows that the link bandwidth is enhanced by both the contributions from narrow laser linewidth and favorable chirp–CD interaction. To the best of our knowledge, our results demonstrate for the first time that all OM4, not limited to selected high bandwidth OM4, can meet the link bandwidth requirements for 100/200 G/lane transmission over 200/100 m. The SM MA VCSEL together with OM4 MMF would provide a low-cost and low-power solution for high data rate transmission for data centers in the AI age.