Optimization of mode numbers of VCSELs for small-cell backhaul applications

Optics Communications 347 (2015) 81–87

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Optics Communications journal homepage: www.elsevier.com/locate/optcom

Optimization of mode numbers of VCSELs for small-cell backhaul applications$ I-Cheng Lu a,n, Chia-Chien Wei b, Jin-Wei Shi c, Hsing-Yu Chen a,d, Sheng-Fan Tsai a, Dar-Zu Hsu a,d, Zhi-Rui Wei c, Jhih-Min Wun c, Jyehong Chen a a

Department of Photonics, National Chiao Tung University, Hsinchu 300, Taiwan Department of Photonics, National Sun Yat-sen University, Kaohsiung 804, Taiwan c Department of Electrical Engineering, National Central University, Taoyuan 320, Taiwan d Information and Communications Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan b

art ic l e i nf o

a b s t r a c t

Article history: Received 31 October 2014 Received in revised form 28 February 2015 Accepted 3 March 2015 Available online 5 March 2015

This paper reports optical orthogonal frequency-division multiplexing (OFDM) transmission using 850 nm Zn-diffusion Vertical-Cavity Surface-Emitting Lasers (VCSELs) and multimode fiber (MMF) for small-cell backhaul applications. We also investigated the influence of side mode suppression ratio (SMSR) on the performance of OFDM modulation. By further optimizing the Zn-diffusion conditions and oxide aperture size, a high-power (6.7 mW) SM (SMSR 430 dB) VCSEL is demonstrated. By using OFDM modulation and bit-loading algorithm, record-high BRDP (91 Gb/s km) at 26 Gb/s transmission under FEC threshold (bit error rate (BER)o 3.8
10  3) across 3.5 km OM4 fiber has been successfully demonstrated. & 2015 Elsevier B.V. All rights reserved.

Keywords: Fiber-optical communications OFDM VCSEL Small cell Fiber-optic backhaul

1. Introduction Recent reports have indicated that the compound annual growth rate (CAGR) of mobile data traffic has reached 61% [1]. Small cells have become one of the most important technologies in mobile data networks due to their significant increase in capacity [2,3]. Short-range (  2 km) fiber-based backhaul technology has been viewed as the most cost-effective [4] and energy-efficient [5] solution for small cell-based wireless heterogeneous networks (HetNets). For current/future fiber-based backhaul applications, active optical cables (AOCs) based on 850 nm Vertical-Cavity Surface-Emitting Lasers (VCSELs) and multimode fibers (MMFs) may be the suitable alternative due to their high modulation bandwidth, low fabrication cost and power consumption [6–10]. By using these VCSELs with simple on-off keying (OOK) modulation, transmission at 440 Gb/s over MMFs has been demonstrated. Unfortunately, this requires high-speed optical as well as electrical devices, which would inevitably lead to an increase in cost. Notably, aiming at 400 Gb/s Ethernet, orthogonal frequency-division ☆ This work was supported by the National Science Council of the Republic of China, Taiwan, under Contract NSC 101-2221-E-009-011-MY3, and Contract NSC 102-2221-E-009-153-MY3. n Corresponding author. E-mail address: [email protected] (I.-C. Lu).

http://dx.doi.org/10.1016/j.optcom.2015.03.004 0030-4018/& 2015 Elsevier B.V. All rights reserved.

multiplexing (OFDM) modulation has been reported as the most cost-effective solution (100 Gb/s
4 channels) with regard to reducing optical transmitter/receiver costs [11]. Moreover, compared with pulse-amplitude modulation-4 (PAM-4) and OOK modulation, OFDM modulation usually has narrower modulation bandwidth, thereby the high-speed integrated circuits (ICs) ( 450 GHz) such as wide-band VCSEL driver and photo-receiver circuit may be not necessary. This can reduce the package and component cost of the full AOC module. On the other hand, aiming at 400 Gb/s Ethernet, the transceiver based on OFDM modulation has shown the reasonable power consumption although it needs more complex signal processing ICs [12]. Remarkable advances in highperformance digital-to-analog converters (DACs) and analog-todigital converters (ADCs) has led to considerable reductions in power consumption and greatly increased their availability [13,14]. This has made OFDM modulation a feasible solution for future AOC applications. By using a 25G-class VCSEL, optical OFDM transmission at 460 Gb/s in the optical back-to-back (OBTB) configuration has been demonstrated [15]. However, the transmission bit rate drops substantially as the fiber distance increases, due mainly to the chromatic and modal dispersion of MMFs [15]. It should be noted that the fact that larger modulation bandwidth ( 440 GHz) induces more serious dispersion effect, which is the main limitation in the bit-rate distance product (BRDP). By narrowing down the optical spectral width, single-mode (SM) VCSELs have been

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reported to boost the BRDP [8,16,17]. In the case of small aperture VCSELs, reducing the diameter of the oxide aperture (typically 2– 3 mm) is an effective means to suppress the higher order transverse mode, which consequently narrows the laser linewidth, and reduce the power consumption as discussed in [8]. However, small aperture VCSEL is usually limited in terms of output power, which might be insufficient for optical links exceeding 2 km. High-power Zn-diffusion SM VCSEL is an another means of boosting the value of the BRDP [17]. Compared with the other reported SM VCSEL structures, it exhibits much lower differential resistance and higher single-mode power. It also eliminates the need for delicate etching processes associated with the top DBR layers. Nevertheless, there is usually a trade-off between VCSEL's modulation speed and its output optical spectral width [17]. The low-frequency roll-off induced by serious spatial hole burning in the SM VCSEL usually limits modulation speed [17–19]. In this work, we began with the fabrication of a Zn-diffusion multimode (MM) VCSEL in order to investigate the impact of side mode suppression ratio (SMSR) on the transmission performances of OFDM modulation. Increasing the bias current (1.9–4 mA) led to a substantial decrease in transmission bit rate when the OM4 fiber was increased to 2 km. This can be attributed to the decrease in SMSR (10.2–5.3 dB). By further optimizing the Zn-diffusion conditions and oxide aperture size, a high-power (6.7 mW) SM (SMSR 430 dB) VCSEL was demonstrated. Despite the fact that the SM VCSEL has a 3 dB electrical-to-optical (E–O) bandwidth narrower than that of the MM one (12 vs. 18 GHz), it provides superior transmission performance by use of OFDM modulation format. Record-high BRDP (91 Gb/s km) at 26 Gb/s transmission under FEC threshold (bit error rate (BER)o3.8
10  3 [20]) across 3.5 km OM4 fiber has been successfully demonstrated by use of OFDM modulation and bit-loading algorithm [21]. Despite the fact that excellent single-mode performance is gained at the cost of a slight decrease in modulation speed, our OFDM transmission results suggest that single-mode performance (SMSR and power) plays a far more important role than modulation speed in maximizing BRDP values.

2. Structure and characteristics of optical transmitter Fig. 1(a) and (b) presents the conceptual cross-sectional and top-down views of the MM VCSEL, respectively. As shown in Fig. 1 (a), the epi-layer structure of the MM VCSEL is composed of three In0.15Al0.1Ga0.75As/Al0.3Ga0.7As multiple-quantum-wells (MQWs) sandwiched between 30-period n-type and 20-period p-type Al0.9Ga0.1As/Al0.12Ga0.88As distributed-Bragg-reflector (DBR) layers with an Al0.98Ga0.02As layer just above the MQWs for oxidation. Fig. 2(a) and (b) presents the conceptual cross-sectional and topdown views of the SM VCSEL, respectively. The epi-layer structure for the SM VCSEL is composed of three GaAs/Al0.3Ga0.7As MQWs sandwiched between 30-period n-type and 20-period p-type

WZ

Metal pad

P contact DZ

Zn-diffusion

PMGI Oxide relief InAlGaAs/AlGaAs MQWs

N contact n-DBR

WZ

Metal pad

P contact

DZ

Zn-diffusion Oxide relief GaAs/AlGaAs MQWs

PMGI

N contact n-DBR Substrate WO Mesa

Fig. 2. (a) Cross-sectional view and (b) top-down view of SM VCSEL.

Al0.9Ga0.1As/Al0.12Ga0.88As DBR layers with an Al0.98Ga0.02As layer just above the MQWs for oxidation. The entire MM and SM VCSELs structures are grown on GaAs substrates. Oxide-relief and Zn-diffusion techniques are used for both the MM and SM VCSELs. The oxide-relief process is used to reduce parasitic capacitance and thereby improve the RC-limited bandwidth [10]. The Zn-diffusion process is applied to the top DBR of the VCSEL to reduce device resistance and manipulate the number of transverse modes [10,17,18]. Lasing modes are suppressed by the Zn-diffused DBR region. The threshold gain becomes larger due to an increase in free-carrier absorption loss and a decrease in reflectivity associated with the diffusion (DBR disordering) process [10,18]. The Zn-diffusion and oxide-relief process are explained in more detail in [10,17,18]. During the device fabrication, there are three important parameters that can be controlled: DZ, WZ, and WO, representing the Zn-diffusion depth, aperture diameter, and the oxide-relief aperture diameter, respectively. Table 1 lists the cavity length and the geometric size of the MM and SM VCSELs. Fig. 3(a) and (b) illustrates the power–current–voltage (L–I–V) characteristics of the MM and SM VCSELs, respectively. Fig. 3 (c) illustrates the differential resistance versus bias current of the MM and SM VCSELs. As can be seen, by further increasing Zndiffusion depth (DZ), the SM VCSEL has higher threshold current (2.1 mA vs. 0.4 mA) and smaller resistance (84 Ω vs. 4 200 Ω) as compared with the MM VCSEL. Such results are consistent with our previous results [10]. Compared with the MM VCSEL, the SM VCSEL also presents greater saturation power (6.7 mW vs. 2.5 mW) due to an increase in oxide aperture diameter [10]. Fig. 4 (a) and (b) presents the measured E–O responses of the MM and SM VCSELs under various bias currents, respectively. The 3 dB bandwidths (f3dB) of the MM and SM VCSELs are specified in Fig. 4 (a) and (b), respectively. The E–O response was determined using a vector network analyzer (Anritsu 37397C) integrated with a calibrated photoreceiver module (New focus 1481S; 22 GHz 3 dB bandwidth). During VNA calibration, the optical-to-electrical (O–E) response of the reference photoreceiver module was carefully deembedded. Fig. 4(c) presents the f3dB versus ratios of bias currents to threshold currents of the MM and SM VCSELs. As can be seen, the damping effect becomes increasingly pronounced with an increase in bias current. This can be attributed to an increase in the gain compression induced by device-heating, thereby the maximum f3dB saturates. Such phenomena are very common in Table 1 Geometric size of studied devices. Device

DZ (mm)

WZ (mm)

WO (mm)

MM VCSEL SM VCSEL

1 2

6 6

4 8.5

Substrate WO Mesa Fig. 1. (a) Cross-sectional view and (b) top-down view of MM VCSEL.

DZ: depth of Zn-diffusion aperture. WZ: width of Zn-diffusion aperture.; WO: width of oxide-relief aperture.

3.5

3.5 L-I curve I-V curve

2.5

2.5

2.0

2.0

1.5

1.5

MM VCSEL 850 nm 25°C Ith = 0.4 mA

0.5 0.0

0

1

6

3 4 5 6 Current (mA)

7

8

9

2.5 2.0 1.5

3

SM VCSEL 850 nm 25°C Ith = 2.1 mA

2

0

2

4

-3 -6

MM VCSEL 850 nm 25°C

-9 -12

1.9 mA, 16.3 GHz 3 mA, 17.9 GHz 4 mA, 18.3 GHz

-15 -18 0

3.0

4

0

0.0

0

3.5

5

1

1.0

12

14

0.0

MM VCSEL SM VCSEL

450 400 300 250 200

3 mA

230

4 mA

212

4

6

8

10 12 14 16 18 20 22

6 3

MM VCSEL Rdiff (Ω) 1.9 mA 261

350

2

Frequency (GHz)

0.5

6 8 10 Current (mA)

500

Rdiff (Ω)

0.5

L-I curve I-V curve

7

Power (mW)

2

1.0

3

EO response (dB)

1.0

Voltage (V)

3.0

83

6

Voltage (V)

Power (mW)

3.0

Normalized response (dB)

I.-C. Lu et al. / Optics Communications 347 (2015) 81–87

0 -3 -6 -9

SM VCSEL 850 nm 25°C

-12

4mA, 4.7 GHz 8mA, 11.4 GHz 12mA, 12.1 GHz

-15 -18 0

2

4

6

8

10

12

14

6

7

Frequency (GHz)

150 100

SM VCSEL Rdiff (Ω)

50 0

8 mA 0

1

2

20

84 3

4

5

6

7

8

Current (mA)

semiconductor lasers [22–24]. As shown in Fig. 4(b), the low-frequency roll-off induced by spatial hole burning effects in typical SM VCSELs [22–24] is not significant in our proposed SM VCSEL structure. Such excellent performance (f3dB 4 12 GHz) can be attributed to the optimization of the cavity design (DZ, WZ, and WO) [25–27]. We also evaluated the maximum intrinsic 3 dB bandwidths (K-parameters) of the MM and SM VCSELs using the extracted RC-limited frequency response. This was obtained from the measured microwave reflection coefficient (S11) and simulated intrinsic frequency responses based on the small-signal solution of laser rate equations [23]. The extracted K-parameters for the MM and SM VCSELs are 0.377 and 0.56 ns, respectively, which correspond to maximum f3dB values of 23.6 and 15.9 GHz. Fig. 5 (a) presents the optical spectra of the MM VCSEL measured by an optical spectrum analyzer (Advantest Q8384) with resolution bandwidth of 0.01 nm under various bias currents. The SMSR of the MM VCSEL under various bias currents are also specified in

16

f3dB (GHz)

Fig. 3. L–I–V characteristics of (a) MM VCSEL and (b) SM VCSEL. (c) Differential resistance of MM VCSEL and SM VCSEL under various bias currents. (MM VCSEL: DZ/WZ/WO ¼1/6/4 mm; SM VCSEL: DZ/WZ/WO ¼2/6/8.5 mm).

MM VCSEL SM VCSEL

18

9

14 12 10 8 6 4

1

2

3

4

5

I/Ith (times) Fig. 4. E–O frequency response of (a) MM VCSEL and (b) SM VCSEL under various bias currents. (c) f3dB versus ratios of bias currents to threshold currents of MM and SM VCSELs. (MM VCSEL: DZ/WZ/WO ¼ 1/6/4 mm; SM VCSEL: DZ/WZ/WO ¼2/6/8.5 mm)

Fig. 5(a). In the MM VCSEL, we observed a red shift and degradation in SMSR with an increase in bias current. Fig. 5(b) presents specific values of power levels associated with the MM VCSEL in

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Relative intensity (dB)

1-Vpp signal -27.17 120 4mA -30 100 MM VCSEL 3mA ° 80 850 nm 25 C -37.41 1.9mA -40 60 SMSR=5.3 dB 40 -50 20 0 SMSR=7.6 dB -20 -60 -40 -60 SMSR=10.2 dB -70 -80 838 840 842 844 846 848 850 844 Wavelength (nm)

Signal Generator MMVCSEL: 2 km OM4 fiber SMVCSEL: 2.3 or 3.5 km OM4 fiber 50 GS/s Real-Time Scope 846

Fig. 5. (a) Optical spectra and SMSR of MM VCSEL at 1.9, 3, and 4 mA. (b) Specific power levels of MM VCSEL in major mode and adjacent mode at 1.9 mA. (MM VCSEL: DZ/WZ/WO ¼ 1/6/4 mm).

major mode and adjacent at 1.9 mA. Fig. 6(a) presents the optical spectra measured using the same optical spectrum analyzer with resolution bandwidth of 0.01 nm under various bias currents. It should be noted that the red shift of the SM VCSEL is slightly smaller than that of the MM VCSEL (0.32 vs. 0.38 nm/mA), which could be attributed to smaller device resistance and/or better heat-sinking. The SMSR of the SM VCSEL under various bias currents are specified in Fig. 6(a). As can be seen, highly single-mode (SMSR 4 30 dB) performance can be achieved over the full range of bias current. Fig. 6(b) presents specific values of power levels associated with the SM VCSEL in major mode and adjacent mode at 8 mA.

3. Experiment setup and results Fig. 7 illustrates the experiment setup of OFDM transmission. The baseband electrical OFDM signal was generated using Matlabs (offline) with the following parameters: fast Fourier transform (FFT) size of 512 and cyclic prefix (CP) of 3.03%; and the bandwidths of 4 GHz and 4.9 GHz for the MM and SM VCSEL, respectively. The electrical OFDM signal was carried out by an arbitrary waveform generator (AWG, Tektronixs AWG7122) at a sampling rate of 10 GSample/s with a digital-to-analog (D/A) conversion resolution of 8 bits and output peak-to-peak voltage (Vpp) of 1 V. The OFDM signal was then fed to the MM or SM VCSEL through a 26.5 GHz bias-T (Agilent 11612A). The output light was

-7.57

Relative intensity (dB)

50

-10

12mA 8mA 4mA

0

SMSR = 31.5 dB

SM VCSEL 850 nm 25°C

-20

-30

SMSR = 37.2 dB

-50

-40 -44.78

-100

SMSR = 36.6 dB

846

848

850

852

-50

854

MM VCSEL: 1.9-4 mA SM VCSEL: 8 mA

849 850 851

Wavelength (nm) Fig. 6. (a) Optical spectra and SMSR of SM VCSEL at 4, 8, 12 mA. (b) Specific power levels of SM VCSEL in major mode and adjacent mode at 8 mA. (SM VCSEL: DZ/WZ /WO ¼ 2/6/8.5 mm).

Photoreceiver MM VCSEL: -9 dBm SM VCSEL: -4 or -7.6 dBm Fig. 7. Experiment setup.

butt-coupled into a lensed fiber with core diameter of 62.5 mm, of which the coupling efficiency was approximately 50%. After OM4 fiber transmission, the optical OFDM signal was directly detected using a 30 GHz photoreceiver (VIS R40-850). The fiber length and received power of the MM and SM VCSELs are separately specified in Fig. 7. The OFDM signal was retrieved and digitized using a realtime oscilloscope (Tektronixs DPO71254) at a sampling rate of 50 GSample/s with a 3 dB bandwidth of 12.5 GHz. Demodulation was performed using off-line Matlabs DSP programs, after which the signal-to-noise ratio (SNR) was estimated, and the BER was measured based on a bit-by-bit comparison between transmitted and received data. It is well known that transmission systems usually suffer uneven channel response [28]. On the other hand, the system performance (i.e., BER) is largely determined by subcarriers with poor performance. By combining OFDM modulation with bit-loading algorithm, we can maximize the bit rate and maintain the counted BER lower than the FEC threshold of 3.8
10  3 [20]. It should be noted that we used the rectangular 8-QAM rather than the circular 8-QAM due to its simpler implementation of decision regions. Fig. 8(a) plots the SNR versus frequency of the OFDM signal with bit-loading algorithm using the MM VCSEL in the optical back-toback (OBTB) configuration under various bias currents. Fig. 8 (b) plots the corresponding modulation levels. The BERs and bit rates are specified in Fig. 8(a) and (b), respectively. As can be seen, the SNR values and modulation levels in OBTB configuration are nearly identical in the case with the three bias currents. Such results may be due to similarities in the E–O frequency responses within a range of 0–4 GHz, as illustrated in Fig. 3. After 2 km OM4 fiber transmission, the SNR degraded with an increase in bias current, as shown in Fig. 8(c). This is because the SMSR degrades as the bias current increases (Fig. 4). These OFDM transmission results suggest that increasing the SMSR and ultimately achieving single-mode performance may be the key to extending the transmission distance (4 2 km OM4 fibers) and achieving a higher BRDP. In other words, single-mode performance is likely to be one of the most important characteristics of VCSELs with regard to MMF transmission (4 2 km). Fig. 8(d) presents the corresponding modulation levels obtained using the MM VCSEL over 2 km OM4 fibers. The BERs and bit rates are also specified in Fig. 8(c) and (d), respectively. Fig. 9(a) presents the SNR versus frequency of the OFDM signal with bit-loading algorithm using the SM VCSEL in OBTB configuration and over 2.3 and 3.5 km OM4 fibers. Fig. 9 (b) plots the corresponding modulation levels. The BERs and bit rates are specified in Fig. 9(a) and (b), respectively. As can be seen, the optical MMF transmission penalty is very small, thanks to the excellent single-mode performance of the SM VCSEL. By the use of our high-power single-mode VCSEL, we have successfully achieved a record-high BRDP of  91 Gb/s km (3.5 km
26 Gb/s) with BER

I.-C. Lu et al. / Optics Communications 347 (2015) 81–87

30

30

MM VCSEL 850 nm 25°C

20 1.9 mA

15

1.9 mA, 6.2x10-4 (-9 dBm) 3 mA, 6.9x10-4 (-9 dBm)

10

SM VCSEL 850 nm 25°C

25

SNR (dB)

SNR (dB)

25

20 15

OBTB, 1.3x10-3 (-4 dBm)

4 mA, 7x10-4 (-9 dBm)

5

0

1

2 3 Frequency (GHz)

10

4

7 6 5 4

1.9 mA, 22.5 Gb/s (-9 dBm) 3 mA, 22.5 Gb/s (-9 dBm) 4 mA, 22 Gb/s (-9 dBm)

3 2 0

1

30

2 3 Frequency (GHz)

4

1.9 mA, 2.1x10-3 (-9 dBm) 3 mA, 1.3x10-3 (-9 dBm)

25 SNR (dB)

0

1

2

6 5 4

OBTB, 26.5 Gb/s (-4 dBm) 2.3 km, 26.5 Gb/s (-4 dBm) 3.5 km, 26 Gb/s (-7.6 dBm)

3 2 0

1

2

3

7

MM VCSEL 850 nm 25°C

6 5

FEC limit: 3.8x10-3

BER

4

1.9 mA, 17.8 Gbps (-9 dBm) 3 mA, 8.6 Gbps (-9 dBm) 4 mA, 7.1 Gbps (-9 dBm)

8

1E-3

SM VCSEL 850 nm 25°C -11 -10

4

5

OBTB (26.5 Gb/s) 2.3 km (26.5 Gb/s) 3.5 km (26 Gb/s)

0.01

10 2 3 Frequency (GHz)

4

Frequency (GHz)

15

1

5

7

MM VCSEL 850 nm 25°C

50

4

SM VCSEL 850 nm 25°C

8

4 mA, 1.3x10-3 (-9 dBm)

20

3

Frequency (GHz)

MM VCSEL 850 nm 25°C

Modulation Level (bit/Hz)

Modulation Level (bit/Hz)

8

2.3 km, 1.5x10-3 (-4 dBm) 3.5 km, 3.5x10-3 (-7.6 dBm)

5

Modulation Level (bit/Hz)

85

-9

-8

-7

-6

-5

-4

-3

-2

Receiver Power (dBm)

3

Fig. 9. (a) SNR and (b) modulation levels versus frequency of SM VCSEL over various lengths of OM4 fibers. (c) BER versus received power of SM VCSEL over various lengths of OM4 fibers. (SM VCSEL: DZ/WZ/WO ¼2/6/8.5 mm).

2 0

1

2 3 Frequency (GHz)

4

Fig. 8. (a) SNR and (b) modulation levels versus frequency of MM VCSEL under various bias currents in OBTB configuration. (c) SNR and (d) modulation levels versus frequency over 2 km OM4 fibers under various bias currents. (MM VCSEL: DZ/WZ/WO ¼1/6/4 mm).

of o3.8
10  3. Fig. 9(c) plots the BER versus received power using the SM VCSEL in OBTB configuration and over 2.3 and 3.5 km OM4 fibers. The energy to data-distance ratio (EDDR, fJ/bit/km) is commonly used to evaluate power consumption when transmission

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4QAM

32QAM

8QAM

64QAM

8QAM

64QAM

16QAM

128QAM

16QAM

128QAM

256QAM

32QAM

256QAM

Fig. 10. Constellations for (a) MM VCSEL at 1.9 mA over 2 km OM4 fiber and (b) SM VCSEL at 8 mA over 3.5 km OM4 fiber. (MM VCSEL: DZ/WZ/WO ¼ 1/6/4 mm; SM VCSEL: DZ/WZ/WO ¼ 2/6/8.5 mm)..

distances exceed 1 km [29]. It should be noted that this does not include the power consumption of electrical ICs. Although the EDDR of the SM VCSEL is larger than that of the MM one (210 vs 107 fJ/bit/ km), it presents very good transmission performance due to the high-power (4.7 mW at 8 mA) and single-mode (SMSR437 dB) characteristics, which could very possibly extend transmission distances and thereby reduce EDDR. Fig. 10(a) and (b) depict the constellations for the MM and SM VCSELs across 2 km and 3.5 km OM4 fibers, respectively.

4. Conclusions In summary, this paper reports optical OFDM transmission using two types of 850 nm Zn-diffusion VCSELs. Compared with the MM VCSEL, the SM VCSEL presents a superior transmission performance using OFDM modulation thanks to its high-power (6.7 mW) and single-mode (SMSR 4 30 dB) performance. By using OFDM modulation and bit-loading algorithm, record-high BRDP (91 Gb/s km) transmission at 26 Gb/s under FEC threshold (BER o3.8
10  3) across 3.5 km OM4 fiber has been successfully achieved. Achieving such excellent single-mode performance involved a slight sacrifice in modulation speed; however, our OFDM transmission results suggest that single-mode performance may be far more important than high modulation speed in the pursuit of high BRDP.

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