Relative intensity noise of temperature-stable, energy-efficient 980 nm VCSELs [original]

AIP Advances 7, 025107 (2017); https://doi.org/10.1063/1.4974258 7, 025107

Relative intensity noise of temperature-

stable, energy-efficient 980 nm VCSELs

Cite as: AIP Advances 7, 025107 (2017); https://doi.org/10.1063/1.4974258

Submitted: 18 November 2016 . Accepted: 03 January 2017 . Published Online: 16 February 2017

Hui Li , Philip Wolf, James A. Lott , and Dieter Bimberg

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AIP ADVANCES 7, 025107 (2017)

Relative intensity noise of temperature-stable,

energy-efficient 980 nm VCSELs

Hui Li,1,aPhilip Wolf,2James A. Lott,2and Dieter Bimberg2,3

1Optoelectronic Materials and Technologies Engineering Laboratory of Shandong, College of

Mathematical and Physical Sciences, Qingdao University of Science and Technology, Qingdao,

People’s Republic of China

2Institut f¨

ur Festk¨

orperphysik and Zentrum f¨

ur Nanophotonik, Technische Universit¨

at Berlin,

Hardenbergstrasse 36, 10623 Berlin, Federal Republic of Germany

3King Abdul-Aziz University, Kingdom of Saudi Arabia

(Received 18 November 2016; accepted 3 January 2017; published online 16 February 2017)

The relative intensity noise (RIN) of temperature-stable, energy-efficient oxide

confined vertical-cavity surface-emitting lasers (VCSELs) have been investigated.

Low energy consumption data transmission is achieved by using small oxide-

aperture diameter VCSELs biased at small currents. We demonstrate that energy

efficiency is not in conflict with our VCSELs’ RIN characteristics. The exper-

imental results indicate that small oxide-aperture diameter VCSELs, which are

most suitable for energy-efficient, temperature-stable operation, exhibit lower laser

RIN due to less mode competition inside the smaller optical cavity volume.

Our energy-efficient VCSELs fulfill the RIN requirements of the 32G Fibre

wise noted, is licensed under a Creative Commons Attribution (CC BY) license

(http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4974258]

Vertical-cavity surface-emitting lasers (VCSELs) are the typical light sources used for high-speed

optical interconnects (OIs) based on multi-mode optical fibers (MMF). The attributes of VCSEL-

based OIs include low energy consumption, low cost, efficient optical coupling, and direct current

modulation.1–3High speed and energy efficiency of OIs is achieved by directly modulating small

oxide-aperture diameter VCSELs biased at moderate currents with simple non-return-to-zero (NRZ)

on-off keying data coding.4–6This modulation format is limited by the analog –3-dB bandwidths

of the VCSELs. The VCSEL’s relative intensity noise (RIN), which is a major source of OI system

noise, degrades the signal quality and increases the bit error ratio (BER) under certain conditions.7

A low RIN floor across the operating range is needed to achieve a large signal-to-noise ratio (SNR).

RIN characteristics often get less attention in device research, where the focus is on the modula-

tion characteristics. System limitations due to poor RIN are often compensated by over-driving the

VCSELs, getting larger modulation bandwidth than needed as compared to a system with better RIN

characteristics.8This strategy increases strongly the required operating power and is in contrast to

efficiency optimized approaches.

We previously performed systematic investigations of 980 nm VCSELs in data transmission

experiments focusing on energy efficiency.9,10 The results show that energy efficiency strongly

depends on the oxide-aperture diameter.10 The smallest possible heat-to-bit-rate ratio, defined as

HBR = (Pel –Popt)/BR, where Pel =I·Vis the input continuous wave (CW) electrical power due

to a bias current Iand a corresponding bias voltage V,Popt is the optical output power, and BR is

the bit rate,11 is usually achieved by using small oxide-aperture diameter VCSELs biased at low

currents. Since RIN is also strongly dependent on the oxide-aperture diameter and bias current, the

RIN characteristics for energy-efficient VCSELs are important for overall OI system performance.

In this paper, we perform a detailed RIN analysis of our energy-efficient, temperature-stable 980 nm

VCSELs.

aElectronic mail: [email protected]

2158-3226/2017/7(2)/025107/5 7, 025107-1 ©Author(s) 2017

025107-2 Li et al. AIP Advances 7, 025107 (2017)

The device structure of our VCSELs is described in detail in Ref. 10 and summarized as follows.

The active region contains five 4.2 nm-thick compressively strained In0.21Ga0.79As quantum wells

(QWs) with 6 nm-thick GaAs0.12P0.88 tensile strained barrier layers that partially compensate the

compressive QW strain. A −15 nm room temperature QW gain peak wavelength to etalon wavelength

offset is employed to improve our VCSEL’s high temperature static and dynamic performance.10

Dry-etched bisbenzo-cyclobutene (BCB) is used to produce a flat and thick (>8µm) dielectric layer

to reduce the parasitic pad capacitance as well as to enable coplanar ground-signal-ground (GSG)

contact pads to avoid parasitic coupling at the probe tip and thus to improve the accuracy of our high-

frequency probing. Details of the static and high bit rate small-signal modulation results are given in

Ref. 10. Oxide-aperture diameters of 3 to 4 µm were observed to be most suitable for energy-efficient

data transmission. Table Isummarizes the results for the VCSELs having oxide-aperture diameters

of 3, 3.5, and 4 µm. To study the noise performance of energy-efficient devices, these 3 to 4 µm

Oxide-aperture diameters VCSELs are used for the present RIN investigations.

Semiconductor laser RIN is a measure of the relative amplitudes of the optical power fluctuations

around the average optical power level, and is quantified by normalizing the noise power by the average

power level. Optical power is detected using a large bandwidth photodetector. Thus, the optical power

fluctuations are transformed to electrical power fluctuations, which are measured with an electrical

spectrum analyzer (ESA). The RIN can be expressed through the electrical values:

RIN =Ntotal

Pavg,elec

=Nlaser +Nth +NPD,shot

Pavg,elec

dBHz(1)

where Ntotal (dBm/Hz) is the overall noise and Pavg,elec is the average electrical power. The overall

noise Ntotal has three noise components: the VCSEL noise Nlaser, the thermal noise Nth, and the shot

noise of the detector NPD,shot. The overall noise (i.e. the total amplified system noise power spectrum)

Ntotal is measured by the ESA with the VCSEL laser diode on. By turning off the laser diode and

keeping the operation of the photodetector and amplifiers unchanged, the electrical spectrum analyzer

measures only the thermal noise power spectrum Nth(f). The frequency-dependent total amplifier

power gain G(f) of two cascaded amplifiers is measured using a 40-GHz Network Analyzer, and is

subtracted from the measured amplified system noise power spectrum to compensate for the gain

and the frequency response of the amplifiers. NPD,shot = 2qIphRLis the shot noise power of the

photodetector under the average VCSEL input power P0, while Iph is the photocurrent out of the

photodetector, and RLis the load resistance of amplifier 1’s input port (see Fig. 1). The shot noise

NPD,shot appears at the photodetector and rises proportionally with the detected optical power. The

average electrical power is given by Pavg,elec=Iph2RL. After subtracting the system thermal noise and

the photodetector shot noise, the intrinsic laser diode RIN can be extracted as:

RIN |laser (f)=Ntotal (f)−Nth (f)/G(f)−NPD,shot

Pavg,elec

(2)

The RIN is characterized with the measurement setup shown in Fig. 1. The VCSEL under test

is biased at a constant current, and the optical output is directly coupled into a 5 m-length lensed

multimode fiber. The OM2 optical fiber is connected to a fast photodetector (a New Focus 1434-50

having a bandwidth of 25 GHz), and the electrical signal is amplified by two cascaded RF broadband

amplifiers (an SHF 100AP and an SHF 804EA) with 19 and 20 dB gain, respectively, to produce

enough amplification to raise the signal above the noise floor of our Hewlett-Packard 8562A electrical

TABLE I. Comparison of room temperature energy efficiency for 3, 3.5, and 4 µm oxide-aperture diameter 980 nm VCSELs.

Parameter 3 µm 3.5 µm 4 µm

Temperature (◦C) 25 25 25

HBR (fJ/bit)a@35Gbit/s 145516151755

HBR (fJ/bit)a@38Gbit/s 14710 18110 19710

I (mA) @35Gbit/s 2.7 3.0 3.3

I (mA) @38Gbit/s 2.9 3.5 3.78

aHBR: heat-to-bit rate ratio.

025107-3 Li et al. AIP Advances 7, 025107 (2017)

FIG. 1. Experimental setup for the RIN measurements.

spectrum analyzer. The New Focus photodetector has a built-in bias monitor to measure the average

photocurrent during the measurements. The resolution bandwidth of the ESA is set to 30 kHz for the

highest possible combination of measurement precision and sensitivity.

Figure 2shows the VCSEL RIN versus frequency of a 3 µm oxide-aperture diameter 980 nm

VCSEL at different bias currents. Note that the RIN peak is suppressed from −116 to −129 dB/Hz by

increasing the bias current from 1 to 2 mA. It is well known that the decrease of RIN upon an increase

in bias reflects the fact that the predominant source of RIN is spontaneous emission. Above threshold,

the noise power does not increase very much, while the optical power rapidly increases. The RIN

reaches a minimum level of −152 dB/Hz at a bias of 4 mA. When the bias current is increased above 4

mA the RIN peak gradually decreases to the shot noise limit. From the measured photocurrent of 0.55

mA, the shot noise level is calculated to be −152.3 dB/Hz. Fig. 2(b) shows the maximum RIN value at

different currents. To indicate the achievable speed, the –3-dB bandwidths obtained from frequency

response measurements at corresponding bias currents are also shown. For the bias current of 3 mA,

the bandwidth reaches a maximum value of 19.9 GHz where the RIN is as low as –141 dB/Hz. The

shot noise limit can be reached by further increasing the forward bias. These RIN data show that our

980 nm VCSELs can be operated at high bit rates with low noise. To realize energy-efficient data

FIG. 2. Measured RIN spectra (a) for different bias conditions at room temperature for a 3 µm oxide-aperture diameter 980-nm

VCSEL and (b) the maximum value of RIN spectra and the –3-dB bandwidth versus bias current.

025107-4 Li et al. AIP Advances 7, 025107 (2017)

FIG. 3. Measured RIN spectra (a) for different bias conditions at room temperature for a 4 µm oxide-aperture diameter 980-nm

VCSEL and (b) the maximum value of RIN and the –3-dB bandwidth versus bias current.

transmission, VCSELs are operated at the lowest currents, where still error-free data transmission is

observed. Fig. 2(a) shows that a low bias current usually corresponds to a large RIN value, so it is

necessary to know whether the RIN is low enough at a given low current energy-efficient operating

point. Table Ishows that error-free data transmission at 35 and 38 Gb/s with low energy dissipation

of 145 and 147 fJ/bit is achieved at bias currents of 2.7 and 2.9 mA, respectively. According to the

32G Fibre Channel (32 GFC) standard,12 the laser RIN is required to be below −131 dB/Hz. Fig.

1(b) shows that this RIN requirement can already be meet for currents larger than 2.2 mA. For all

error-free data transmissions larger bias points were used and the RIN requirements of the 32 GFC

standard are fulfilled. Thus, this implies our 980 nm VCSELs can be operated at high bit rates with

low energy dissipation and with low noise.

For a 4 µm oxide-aperture diameter VCSEL the results are shown in Fig. 3. In contrast to the

3µm oxide-aperture diameter VCSEL, whose noise saturates at the shot noise floor when the bias

current is larger than 4 mA, the RIN of the 4 µm oxide-aperture diameter VCSEL is larger due to

increased mode competition. The results indicate that the smaller VCSEL exhibits lower RIN, which

is consistent with the trend of improved energy efficient operation of smaller VCSELs. The photon

density in the active region is larger for 3 µm oxide-aperture diameter VCSELs due to the smaller

optical mode volume. The relaxation resonance frequency and the damping both increase with an

increase in photon density, which in-turn results in lower RIN values for the smaller oxide-aperture

diameter VCSELs. For energy-efficient data transmission at 38 Gb/s, the bias current is 2.9 mA and

3.78 mA as shown in Table Ifor 3 and 4 µm oxide-aperture diameter VCSELs, respectively. The

corresponding RIN values are as low as –141 and –146 dB/Hz, respectively.

In conclusion, we studied the RIN spectra of energy-efficient, temperature-stable 980-nm

VCSELs at different bias currents at room temperature for different oxide-aperture diameters. Low

RIN operation is achieved even at low bias currents as required specifically for energy-efficient,

error-free data transmission operation. We have found that smaller oxide-aperture diameter VCSELs

025107-5 Li et al. AIP Advances 7, 025107 (2017)

exhibit lower laser diode RIN due to less mode competition at a given –3-dB bandwidth. The trend is

consistent with energy-efficient operation. The VCSELs satisfy the bandwidth and RIN requirements

for the 32 GFC Fibre Channel standard. Our small oxide-aperture diameter (3 to 4 µm) VCSELs

not only benefit from a larger –3-dB bandwidth and lower energy dissipation per transmitted bit, but

they also show extremely low noise. being advantageous for short reach optical interconnects in high

performance computers and in board-to-board and chip-to-chip integrated photonics systems.

This work was supported by the German Research Foundation via the Collaborative Research

Center 787, National Natural Science Foundation of China (Grant No. 11647169), the Natural Science

Foundation of Shandong Province (Grant No. ZR2016FB05), the Natural Science Foundation of

Qingdao (Grant No. 16-5-1-8-jch), and the Open Fund of the State Key Laboratory of Luminescence

and Applications.

1A. Larsson, “Advances in VCSELs for communication and sensing,” IEEE J. Sel. Top. Quantum Electron. 17(6), 1552–1567

(2011).

2A. Kasukawa, “VCSEL technology for green optical interconnects,” IEEE Photonics Journal 4(2), 642–646 (2012).

3D. Bimberg, A. Larsson, and A. Joel, “Faster, more frugal, greener VCSELs,” Compound Semic., 22 (2014).

4P. Moser, P. Wolf, G. Larisch, H. Li, J. A. Lott, and D. Bimberg, “Energy-efficient oxide-confined high-speed VCSELs for

optical interconnects,” in proc. SPIE 9001 Vertical-Cavity Surface-Emitting Lasers XVIII, Feb. 2014, 9001103-1–900103-8.

5H. Li, P. Wolf, P. Moser, G. Larisch, A. Mutig, J. A. Lott, and D. Bimberg, “Energy-efficient and temperature-stable 980 nm

VCSELs for 35 Gb/s error-free data transmission at 85 ◦C with 139 fJ/bit dissipated heat,” IEEE Photon. Technol. 26(23),

2349–2352 (2014).

6E. Haglund, P. Westbergh, J. S. Gustavsson, E. P. Haglund, A. Larsson, M. Geen, and A. Joel, “30 GHz bandwidth 850 nm

VCSEL with sub-100 fJ/bit energy dissipation at 25–50 Gbit/s,” Electronics Letters 51(14), 1096–1098 (2015).

7F. Tan, M.-K. Wu, M. Liu, M. Feng, and N. Holonyak, “850 nm oxide VCSEL with low relative intensity noise and 40 Gb/s

error free data transmission,” IEEE Photon. Technol. Lett. 26(3), 289–292 (2014).

8C.-T. Tsai, S. Chang, C.-Y. Pong, S.-F. Liang, Y.-C. Chi, C.-H. Wu, T.-T. Shih, J. J. Huang, H.-C. Kuo, W.-H. Cheng, and

G.-R. Lin, “RIN suppressed multimode 850-nm VCSEL for 56-Gbps 16-QAM OFDM and 22-Gbps PAM-4 transmission,”

Optical Fiber Communication Conference Optical Society of America, 2016: Th4D. 2.

9P. Moser, J. A. Lott, G. Larisch, and D. Bimberg, “Impact of the oxide-aperture diameter on the energy-efficiency, bandwidth,

and temperature-stability of 980-nm VCSELs,” J. Lightw. Technol. 33(4), 825–831 (2015).

10 H. Li, P. Wolf, P. Moser, G. Larisch, J. A. Lott, and D. Bimberg, “Temperature-stable, energy-efficient and high-speed

oxide-confined 980 nm VCSELs for optical interconnects,” IEEE J. Sel. Top. Quant. Electron. 21(6), 1700409-1–9 (2015).

11 P. Moser, P. Wolf, J. A. Lott, G. Larisch, A. Payusov, N. N. Ledensov, and D. Bimberg, “81 fJ/bit energy-[5]to-data ratio

of 850 nm vertical-cavity surface-emitting lasers for optical interconnects,” Appl. Phys. Lett. 98(23), 231106–231106-3,

(2011).

12 Fibre Channel Roadmaps v1.8, Available: http://fibrechannel.org/fibre-channel-roadmaps.html