Photonic generation and distribution of a modulated 60 GHz signal using a directly modulated gain switched laser

Photonic Generation and Distribution of a Modulated 60 GHz Signal Using a Directly Modulated Gain

Switched Laser

H. Shams, P. M. Anandarajah, P. Perry, and L. P. Barry Research Institute for Networks and Communications Engineering (RINCE),

Dublin City University (DCU), Dublin, Ireland.

[email protected]

Abstract— We propose a simple and cost effective approach for the optical generation and downstream transmission of a data modulated 60 GHz signal using a gain switched laser (GSL). Gain switching is achieved by driving the laser with a 15 GHz local oscillator combined with a 1.25 Gbps on-off keyed (OOK) data stream. The system was tested over a 3 km length of fibre and a 2 m radio link and showed good performance. Simulations were also carried out for investigating the impact of optical suppression of unwanted subcarriers on the system performance. The simulations were also performed at up to 15 Gbps and showed the possibility of using the proposed architecture at these high bit rates.

Keywords- Gain-switching, directly modulated laser (DML), radio-over-fiber (RoF), millimeter-wave (MMW) generation.

I. INTRODUCTION The use of millimeter waves (mm-waves) for delivering

high definition television (HDTV) streams and high data rates to the end users has attracted significant interest from researchers over the past few years [1-9]. Radio-over-fiber (RoF) systems have been used to extend the maximum reach of such systems due to its large available bandwidth, high performance and low cost. The diagram in Fig. 1 shows the general topology of such a system where high bit rate video can be distributed from a central source or media gateway to a central station that can manage the network resources and reconfigure the deployed network to satisfy changing traffic demands. This topology enables network reconfiguration in the central station which can be linked to mobility management functions to enable centralized network optimization.

Optical mm-wave generation typically involves the generation of two correlated optical carriers at a central station (CS) with a frequency offset equal to the desired mm-wave frequency [1-7]. The carriers are then transmitted through optical fiber and beat together at the high speed photodetector. One of the most widely used techniques employs external intensity modulators to generate frequency doubling or quadrupling of the driven RF sinusoid signal [4, 5]. However, this technique usually requires two external modulators (one for generating the optical carriers with the required separation and one for data modulation) which increases both signal attenuation and cost, and is more susceptible to bias drifting of

the modulators, which can affect the output spectrum. The direct modulation of a laser has also been used for optical mm-wave generation, but was limited by the laser’s frequency response, and produced very low optical power [6, 7]. Other techniques employed gain switched and pulsed semiconductor lasers to produce a comb of multiple frequency tones, equally spaced by the same drive frequency [8]. These optical modes are phase locked together as they are derived from the same lightwave source and filtering can be used to generate mm-wave signals at different frequencies.

In our recent work we have used the gain switching technique followed by optical filtering and external modulation to generate a 1.25 Gbit/s on a 60 GHz carrier [9]. In this paper, we propose a novel technique based on a gain switched laser (GSL) for optical generation and transmission of mm-wave that does not require any external modulator (Fig.2). This architecture is simple and easy to implement as it does not require any external modulator, and hence there is no additional insertion loss, and cost which is a vital for optical generation and distribution of 60 GHz signals. Gain switching is achieved by driving the laser with a large RF signal. In our new system, the RF signal is produced by coupling the sinusoid signal with non-return to zero (NRZ) data. The GSL generates multiple phase correlated sidebands, spaced by the driving RF

Figure 1. The general network topology for mm-wave distribution over fiber

frequency and modulated with data. By using the appropriate filters to select specific components of the comb, the generated mm-wave signal can be at many times the drive RF signal frequency. The selected sidebands are transmitted over an optical network containing passive optical splitters to a number of remote antenna units (RAUs). These components beat together at the detector to yield an amplitude modulated mm-wave signal. This signal is then amplified and transmitted to the mobile units (MUs) by using an antenna. Using this proposed method, we successfully implemented the transmission of 1.25 Gbps downstream data over 3 km fiber with 2 m wireless distance on a 60 GHz RF carrier. The system performance has been measured by using a bit error rate (BER) detector, and the eye diagrams have been recorded after transmission. This system can easily generate mm-waves with a stable spectrum with less cost and complexity than comparable systems.

The rest of this paper is organized as follows. In section II, we represent the experimental setup and results for our proposed system. Simulation results are presented in section III. Finally, section IV concludes the paper.

II. EXPERIMENTAL SETUP AND RESULTS The proposed system shown in Fig. 3 represents the

experimental setup for optical mm-wave generation and downstream data transmission over the fiber. The CS consists of a commercial distributed feedback laser diode (DFB-LD) with an emission wavelength of 1551 nm at room temperature and a threshold current of 15 mA. The DFB-LD is biased at 43 mA and gain switched by an RF signal. The RF signal is composed of an 15 GHz sinusoidal signal coupled with 1.25 Gbps downstream data (pseudo-random bit sequence with a 27-1 word length) generated by using a pulse pattern generator synchronized with the 10 MHz reference from the signal generator (SG). The generated RF signals are shown as inset (i) in Fig. 2, and it can be seen that each temporal bit slot consists of twelve cycles of the 15 GHz sine wave. The DFB-LD is externally injected with another DFB-LD to decrease chirping and the time jitter of the gain switched pulses [10]. The generated gain switched spectrum is captured by a high resolution optical spectrum analyzer (OSA) and is shown in Fig. 4(a). The spectrum shows the generated comb with 15 GHz spacing between comb lines, and each tone modulated by the 1.25 Gbps data stream. The optical spectrum consists of nine central sidebands covering a spectral range over 120 GHz, with a maximum power difference between the comb lines of about 5 dB. This optical spectrum is filtered by using two optical filters. An optical band stop filter (OBSF) in the form of a fiber Bragg grating with 3 dB bandwidth of 0.28 nm centered at 1550.918 nm was used to suppress three optical tones in the middle and an optical band pass filter (OBPF) with bandwidth 0.485 nm was used to reject the outer sidebands., The resultant output spectrum is illustrated in Fig. 4(b) and it shows the two main optical tones spaced by 60 GHz which are clearly modulated with the data stream.

The suppressed sidebands are shown around 15 dB below than the main sidebands. This level of suppression was limited by

Figure 2. Block diagram of the proposed setup

Figure 3. The experimental setup for mm-wave generation and transmission for downstream link

the optical filters available, and much better suppression could be achieved by employing a specially designed Bragg filter to select the two required sidebands.

This effect is explored later in the simulation section. The filtered optical signal as captured by an oscilloscope is shown as an inset (ii) in Fig. 3. The extinction ratio between 1 and 0 bits was adjusted to give the best performance. This is necessary because as the extinction ratio is increased the level of timing jitter and noise on the optical pulses increases, and this reduces overall system performance.

After filtering, the optical signal is amplified by using an erbium doped fiber amplifier (EDFA) and then transmitted over standard single mode fiber (SSMF) to the RAU. At the RAU, the optical signal is photodetected by a high speed photodiode with a 3 dB bandwidth of 50 GHz. The two sidebands beat together in the receiver and generate a modulated 60 GHz mm-wave. The converted electrical signal was subsequently boosted by a mm-wave amplifier to compensate for the limited bandwidth of the detector. Afterwards, the mm-wave signals are broadcasted to an MU via a 20 dBi horn antenna.

At the MU, the mm-wave signal is received by an identical horn antenna, amplified, and mixed with a 60 GHz LO to down

convert to a base band signal. The signal is then filtered by using a low pass filter (LPF) and amplified again.

The demodulated 1.25 Gbps signal was detected by a bit error rate tester (BERT) and the eye diagrams were monitored by using a high digital speed sampling oscilloscope (OSC). The link power budget was also investigated at the radio system. The equivalent isotropically radiated power (EIRP) at the transmitter was -5 dBm, and the free space loss (FSL) after 2 m wireless distance was calculated using:

FSL (dB) = 92.4 + 20 log(F)+20 log(D)

where F is the frequency in GHz, and D is the line of sight distance in km. With a receive antenna gain of 20 dBi, then, the received electrical power was found to be -59 dBm.

In Fig. 5, the measured BER is plotted versus the received optical power for back-to-back (B2B), and 3 km fiber transmission with and without wireless transmission. The eye diagrams of the recovered baseband signals are also shown as insets in Fig. 5. The inset (i) in Fig. 5 exhibits the eye diagram for B2B without wireless transmission at received power of -30.9 dBm. As can be seen from the figure, the B2B receiver sensitivity for BER of 10-9 is -31.9 dBm and there is 0.7 dB power penalty after 3 km fiber transmission without wireless transmission. For B2B optical connection and 2 m wireless transmission, the receiver sensitivity was degraded by 4.4 dB to -26.5 dBm due to the signal to noise ratio (SNR) degradation in the radio system. For a combined 3 km fiber and 2 m wireless scenario, the receiver sensitivity is further degraded to about -25.2 dBm, and the eye-diagram in this case is shown as inset (ii) in Fig. 5 for a received power -24.4 dBm.

Figure 5. The measured BER for baseband signal versus the received optical power and the eye diagrams for (i) B2B and (ii) 2 m wireless with 3 km fiber

transmission

Figure 4. The measured optical spectrum for (a) direct modulated gain switched laser, and (b) after filtering.

III. SIMULATION RESULTS The simulation model for optical mm-wave generation by

using direct modulation of GSL has also been carried out by VPI TransmissionMaker simulation platform. VPI simulation provides designed photonic devices and components for optical system analysis and evaluation. The simulation parameters were chosen to emulate the real experimental parameters. The direct modulation of the GSL has been achieved by using a transmission line model (TLM) which is able to simulate the full dynamics of laser including its spectral dynamics. The laser output was optimized by changing both the bias and amplitude of the RF signal. A CW laser module was also used to realize the external injection. The fiber link was a 3 km SSMF with a group velocity dispersion (GVD) of 16 ps/nm/km and loss of 0.2 dB/km. A self mixing demodulator was used at the MU to down-covert the modulated mm-wave signal to the baseband.

In this simulation model, The GSL was driven by a signal composed of a 15 GHz sinusoid signal and 1.25 Gbps NRZ data stream. The simulated optical spectrums at the output of

the GSL and after filtration are shown in Fig. 6. There are around 11 optical sidebands generated at 5 dB bandwidth, each of them is modulated with the downstream data.

We first investigated the influence of the optical sideband suppression ratio on the system performance. The optical sideband suppression is defined as the optical power difference between the desired optical component and the middle optical sideband. The optical carrier suppression was achieved by controlling the rejection parameter in the optical fiber Bragg grating filter module. The filtered optical spectrum is shown in Fig. 6(b) with the suppression ratio = 50 dB. The BER versus the received optical power (ROP) was plotted for various optical sideband suppression ratios and is shown in Fig. 7. It can clearly be seen that, as the optical suppression for the sidebands increases, the system performance is improved and the lowest BER curve is obtained. These optical sidebands will propagate inside the fiber with different velocity, then each pair of sidebands at 60 GHz separation can beat together to generate mm-wave signals at different phases which will interfere with the main generated 60 GHz and degrade the SNR of the system. In the experimental setup, the optical suppression for optical tones in our experimental schemes refers to the optical suppression ratio 30 dB in our simulation results.

We also investigated the impact of higher bit rate transmission with a constant 40 dB optical power sideband suppression. The bit rates were chosen to generate an integer number of pulses per bit, so that rates of 3, 5, 7.5, and 15 Gbps consist of 5, 3, 2, and 1 pulses for each bit slot, respectively. However, the tests at 10 Gbps were achieved by driving the laser with 20 GHz signal coupled with 10 Gbps data stream, to give 2 pulses per bit. Fig. 8 shows the eye diagrams for these bit rates after transmission over a 3 km fiber link. The BER versus received optical power for each data rate are also exhibited in Fig. 9. Clearly, the system performance degrades with the increase of data rate, and this is due to the system

Figure 7. The simulated BER versus the received optical power for different suppression ratios

Figure 6. The simulated results for (a) gain switched spectrum, and (b)

filtered spectrum with a suppression ratio = 50 dB

sensitivity on the number of pulses per bit and the timing jitter from the direct modulation of the laser which closes the eye of the received signal. In Fig. 9 the data rate at 15 Gbps shows an error floor at BER<10-7. This could be clearly seen from the distorted eye diagrams in Fig. 8(e). These results show the simplicity and low cost system for realizing the distribution of higher bit rates without using any external modulators.

IV. CONCLUSION In this paper, we have proposed a simple and cost effective

method for the generation and transmission of a 1.25 Gbps downlink stream in a RoF system operating at 60 GHz, based on a directly modulated gain switched DFB-LD. The generated spectrum has relatively flat optical modulated tones spaced by 15 GHz that are phase correlated. Two optical tones have been filtered out to generate a 60 GHz optical mm-wave. The BER performance was evaluated with and without wireless transmission for B2B and over 3 km downlink fiber. The system shows a small power penalty between B2B and 3 km fiber without or with wireless transmission.

In this proposed RoF system, we directly generated the modulated mm-wave signal using frequency quadrupling and no external intensity modulators are required. The system was also simulated to show the impact of the optical suppression of

the filters and the use of higher bitrates. It was noted that the higher optical suppression for the sidebands improves the system performance and achieves lowest BER. The simulation results show that the system was capable of delivering 15Gbps and could possibly be used for transmitting higher data rates. This system reduces the cost of the overall system which is critical for the commercial deployment of short range distribution in home or business buildings.

ACKNOWLEDGMENT This work was supported by the Science Foundation

Ireland Research Frontier Grant and an Enterprise Ireland Technology Development Phase Grant.

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Figure 9. The BER versus received optical power for different bitrates.

Figure 8. The eye diagrams for (a) 3 Gbps, (b) 5 Gbps, (c) 7.5 Gbps, (d) 10 Gbps, and (e) 15 Gbps.

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