Phase and Amplitude Coding Separation Based on

Phase and Amplitude Coding Separation Based on the Injection-Locked Single-Mode VCSEL 1

1,2

Franko Küppers , Vladimir S. Lyubopytov

1

Tuomo von Lerber 1

3

1,3

University of Helsinki, P.O. Box 68 (Gustaf Hällströmin katu 2b), FI-00014 Helsinki, Finland It has been shown earlier, that the injection locked semiconductor lasers enable effective amplitude noise suppression [4] and makes possible an extra level of signal multiplexing – orthogonal modulation [5], where PSK and ASK NRZ channels propagate at the same wavelength [6]. In our work we use an injection-locked VCSEL as a slave laser providing separation of amplitude and phase modulations, carrying independent information flows. VCSELs are known to have high energy efficiency due to extremely small threshold currents, low fabrication cost, possibility of on-chip integration, and effective coupling with an optical fiber. This approach can be used as a cost- and energy-efficient extension to improve the capacity of short-range optical fiber communications and optical interconnects for data centers.

Keywords—Vertical cavity surface emitting lasers; Phase modulation

I.

INTRODUCTION

A laser dynamics (including the dynamics with external seeding) has been rather deeply investigated both theoretically and experimentally [1]. Region of parameters of stable injection locking has been determined [2]. However, in most cases the seeding signal is assumed to be either CW or amplitude modulated. One important subcase of external phase modulation appears when the laser follows the phase of the external seeding, but keeps its own (almost independent of the amplitude of the input seeding signal) output amplitude [3]. PC1

Master Clock 1 GHz

3

, Matti Lassas

Institute for Microwave Engineering and Photonics, Technische Universität Darmstadt, Merckstr. 25, 64283 Darmstadt, Germany 2 Department of Photonics Engineering, Technical University of Denmark (DTU), Ørsteds Plads 343, 2800 Kgs. Lyngby, Denmark

Abstract—Information capacity of commercial channels can be doubled by a simultaneous independent phase and amplitude modulation without coherent detection. Separation of the phase and amplitude modulation has been demonstrated by the use of VCSEL.

ECL

1

, Arkadi Chipouline , Mohammadreza Malekizandi ,

II.

To validate the possibility of amplitude- and phase-coded channels separation by an injection-locked VCSEL, an experimental setup shown in Fig. 1 has been used. OC

PC2

EAM PRBS 1 29–1

PPG1

OPERATION PRINCIPLE AND EXPERIMENTAL SETUP

PM PRBS 2 27–1

60

OF1 VOA1 EDFA1

3 nm

PPG2

40

1

VOA2

3

Slave J=2.8...3.0 mA

VCSEL Pinj= –14…–7 dBm

95

5

PD 2

PC3 2

90

OF2

EDFA2

3 nm

10

PD 3

OSA

DLI

VOA3 PD 1

Output PM signal

Osc.

BERT

Output AM signal

Fig. 1. Scheme of the experimental setup (VOA – variable optical attenuator; OF – 3-nm band pass optical filter; PD – p-i-n photodiode; DLI – delay line interferometer; OSA – optical spectrum analyzer; Osc. – digitizing oscilloscope)

CW light from a tunable external cavity laser (ECL) used as a master laser, is consecutively modulated by amplitude and phase with 1 Gbaud symbol rate. An electro-absorption modulator (EAM) and phase modulator (PM) are independently fed by two pulse pattern generators (PPG), which provide pseudorandom binary sequences (PRBS) with lengths of 29–1 and 27–1 bits respectively. PPGs are synchronized in such a way that the modulated amplitude and phase symbols in optical signal coincide in time. Then the modulated signal is attenuated, which models losses in the optical transmission line. The amplified spontaneous emission (ASE) noise of optical amplifiers is introduced in order to test

different OSNR scenarios in a hypothetic transmission system. After EDFA and 3-nm optical bandpass filter, the received signal passes through the variable attenuator, which allows optimizing an injection power. An optical circulator is used to separate the injected and emitted by VCSEL signals. The VCSEL (slave laser) is pumped slightly above its lasing threshold. Polarization controller before the VCSEL provides the optimal polarization state of the injected optical signal. The output signal of VCSEL after the circulator is detected by a differential binary phase-shift keying (DBPSK) receiver, which consists of EDFA with a 3-nm Band Pass optical Filter (BPF), delay line interferometer (DLI), variable attenuator,

The amplitude disturbance is maximally suppressed when the detuning between master and slave lasers approaches zero (from the shorter wavelengths). An absolute value of detuning was first minimized coarsely by setting the wavelength of ECL as close as possible to the free-running wavelength of VCSEL, and then optimized more precisely by changing the temperature and driving current of the VCSEL. An optimal injection power was empirically adjusted in order to minimize BER in the demodulated signal after DBPSK receiver. III.

damping relaxation oscillations and establishing a steady-state regime. For the VCSEL used in our setup, relaxation oscillations take 0.4 ns, so that the optical signal with a symbol rate not higher than 2.5 Gbit/s can be processed. However, state-of-art VCSELs with enlarged bandwidth [7] could be used to increase the signal processing speed. -1 -3

-6 -7 PM signal component, reference AM signal component PM signal component, with VCSEL

-9 -10 -11 2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

6,5

7,0

7,5

AM Extinction Ratio [dB] Fig. 3. BER vs. extinction ratio of amplitude modulation for the amplitudemodulated channel (blue), and the DBPSK channel before (red) and after (green) phase rectification using VCSEL. Points A and B show the crossing with the FEC BER threshold; crossing points C and C’ are the points of equal BERs for amplitude- and phase-modulated channels.

IV. CONCLUSIONS We have shown that the special operation mode of a seeded VCSEL can be used for effective separation of the phase and amplitude modulated signals simultaneously carried by the same wavelength. We have demonstrated the modulation/demodulation scheme which provides at least double information capacity increase using the standard communication system architecture by the cost of one extra element (VCSEL) per wavelength channel.

10 9 7 6 5 4

6.37 dB

ER=7.8 dB 4.75 dB

AM Extinction Ratio [dB]

C'

-5

-8

11

8

7% OH HD-FEC limit

C

-4

EXPERIMENT RESULTS AND DISCUSSION

Figure 2 demonstrates suppression of the amplitude modulation in the optical signal by the VCSEL. At the value of ER=7.8 dB, where eye diagram of the detected DBPSK-signal tends to be closed, employment of the injection-locked VCSEL provides the gain of 4.75 dB in diminishing ER, which allows keeping the eye open.

B

A

-2

log BER

and p-i-n photodiode. Analysis of the detected bit sequences encoded by the optical signal amplitude and phase is performed by the bit error rate tester (BERT), connected in the former case to the 40%-output of the fiber splitter after EDFA 1, and in the latter case – to the output of DBPSK receiver. Eye diagrams and extinction ratio (ER) measurements are provided with digitizing oscilloscope.

3 2 1 0,8 0,9

1

1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9

2

REFERENCES

Modulating Voltage [V]

[1]

ER before VCSEL ER after VCSEL

[2]

Fig. 2. Experimental setup for waveguide transmission and characterization of the OAM modes.

Figure 3 shows BER vs. AM ER dependences for the amplitude- and phase-modulated signals in the presence of ASE for losses in the transmission line of –12 dB. The utilization of the injection-locked VCSEL provides significant improvement of the transmission reliability for DBPSK signal component – improvement of more than an order is recognized (points C and C’). The use of the injection-locked VCSEL as an optical phase rectifier gains BER with a simple differential phase demodulation scheme even for quite high ERs of amplitude modulation, superimposed over the phase one. The lower BER values are required, the more BER improvement can be achieved. Efficiency of suppression of the amplitude modulation by injection-locked laser depends on its time scale. The maximum treatable symbol rate of AM is limited by the time required for

[3]

[4]

[5]

[6]

[7]

H. Haken, Laser Light Dynamics, Vol. 2 (North-Holland Physics Publishing, 1986). E.K. Lau, L.J. Wong, and M.C. Wu, “Enhanced Modulation Characteristics of Optical Injection-Locked Lasers: A Tutorial,” IEEE J. Sel. Topics Quantum Electron. 15(3), 618-633 (2009). J. L. Boulnois, “Injection Locking of CW Ring Gas Lasers” in Physics of New Laser Sources, N. B. Abraham, F. T. Arecci, A. Mooradian, A. Sona, ed., NATO ASI Series (Springer Science + Business Media New York, 1985), pp. 317-330. A. Fragkos, A. Bogris, D. Syvridis, and R. Phelan, “Colorless Regenerative Amplification of Constant Envelope Phase-Modulated Optical Signals Based on Injection-Locked Fabry–Pérot Lasers,” IEEE Photon. Technol. Lett. 24(1), 28-30 (2012). Y. Zhang, N. Deng, C.-K. Chan, and L.-K. Chen, “A Multicast WDMPON Architecture Using DPSK/NRZ Orthogonal Modulation”, IEEE Photonics Technology Letters, 20, 1479, 2008. A. Fragkos, A. Bogris, and D. Syvridis, “Spectrally efficient and High Extinction Ratio DPSK/ASK Orthogonal Modulation Schemes Based on Injection Locking Limiting Amplifiers”, in ECOC Technical Digest, (OSA, 2012), paper P3.04. D. M. Kuchta A. V. Rylyakov, F. E. Doany, C. L. Schow, J. E. Proesel, C. W. Baks, P. Westbergh, J. S. Gustavsson, and A. Larsson, “A 71Gb/s NRZ Modulated 850-nm VCSEL-Based Optical Link,” IEEE Photon. Technol. Lett. 27(6), 577–580 (2015).