Abstract: Next generation optical access networks will require lowcost lasers in conjunction with network flexibility and higher data rates.This work presents the direct modulation of a low cost tuneable slottedFabry-Perot laser (tuneable over 14nm) with AM-OFDM. Characteristics ofthis dual section laser are presented and transmission of 10Gb/s over 50kmis achieved with this device.
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direct modulation AMO-OFDM transmission by optical injection using monolithically integrated lasers,” IEEEPhoton. Technol. Lett. 24(11), 879–881 (2012).
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1. Introduction
Tuneable lasers are highly desirable for use in cost effective Optical Access Networks (OANs)such as Passive Optical Networks (PONs) because they can simplify network architecture byfacilitating the use of identical Optical Networking Units (ONUs) within the network while
#176950 - $15.00 USD Received 28 Sep 2012; revised 15 Nov 2012; accepted 16 Nov 2012; published 30 Nov 2012(C) 2012 OSA 10 December 2012 / Vol. 20, No. 26 / OPTICS EXPRESS B399
still maintaining colourless operation [1]. However, for the advantages of tuneable lasers tobe exploited in future PONs their complexity and associated cost need to be considerably re-duced given the potentially wide deployment of these networks [2]. In order to reduce the costand footprint of a tuneable transmitter in PONs, it is preferable to use direct modulation. Thistechnique also avoids other problems associated with external modulators such as bias drift, in-sertion loss and polarization dependence. A tuneable slotted Fabry-Perot (FP) laser is employedin this work. Compared to RSOAs the SFP eradicates the requirement for optical filtering at theONU and its’ low cost and ease of manufacture give it an advantage over VCSEL devices[5].
Adaptively Modulated Orthogonal Frequency Division Multiplexing (AM-OFDM) has al-ready been shown as a modulation format suitable for implementation in PONs [3][4]. AM-OFDMs high spectral efficiency is given by its ability to adaptively power and bit load eachoverlapping orthogonal subcarrier depending on the channels frequency response. This, cou-pled with its facilitation of a simple maximum likelihood equalizer in the frequency domainto overcome the effects of chromatic dispersion, makes AM-OFDM an excellent modulationtechnique for use in relatively low bandwidth and cost effective optical systems.
The tuneable SFP along with direct modulation is used to construct a potentially cost effec-tive optical system. The device is first characterised so that conditions leading to single modeoperation can be obtained. Then AM-OFDM is used to maximise the data throughput given thevariable direct modulation bandwidth of the device at these different operating conditions.
2. Laser device
Fig. 1: An SMSR map of the tuneable device. The graduated scale gives the SMSR sothat biasing conditions which result in single mode lasing can be clearly identified.
The laser device used in this work was a dual section SFP laser diode. The slots are etchedinto the top of an otherwise conventional laser ridge waveguide transforming the multimodespectrum of the FP laser into a very high quality single mode device [5]. Increased tuning rangeoperation of the laser is achieved by optimising the laser’s mirror reflectivities in each sectionby employing the Vernier effect to extend the tuning range associated with the limited refractiveindex change with current [6]. This device requires no additional re-growth or processing stepswhen compared with other tuneable devices, which minimises the fabrication complexity. Thedevice is controlled by a Thermo-Electric Cooler (TEC) and both of its sections may be biasedindependently. Wavelength tuning is achieved as single mode operation, defined as Side Mode
#176950 - $15.00 USD Received 28 Sep 2012; revised 15 Nov 2012; accepted 16 Nov 2012; published 30 Nov 2012(C) 2012 OSA 10 December 2012 / Vol. 20, No. 26 / OPTICS EXPRESS B400
Fig. 2: Modulation responses; dashed indicates where the right section was driven.
Fig. 3: Experimental setup with inset of an example received optical spectrum.
Suppression Ratio (SMSR) ≥ 30dB [7], can be attained at various wavelengths depending uponthe biasing of both sections and temperature conditions. To ascertain bias conditions for whichsingle mode lasing is achieved, SMSR and output wavelength were measured for a set of biascurrents to each section ranging from 0 to 80mA in 1mA steps. This process was repeated forvarious temperatures ranging from 2.5◦C to 17◦C. Figure 1 shows such an SMSR map obtainedat a temperature of 12.5◦C with the output wavelength stated for each area of high SMSR (≥30dB). The map shows that the number of operating points is limited and many biasing condi-tions do not result in single mode operation. Work is ongoing to increase the number of availablemodes of this prototype device, including those compatible with ITU grid wavelengths, whileminimising cost so as to maintain its suitability for access networks. Approaches to achieve thisinclude further optimisation of the etched features in order to increase the reflection spectrumfrom the slots and the inclusion of passive sections to allow a wider tuning range to be achieveddue to the larger refractive index changes with current in these regions.
For each set of single mode lasing conditions there was an associated modulation responsefor each section. Figure 2 shows modulation responses for three sets of these conditions andthe modulation bandwidths achieved are around 4GHz. The output power of the device variedbetween -3 and -7dBm depending on biasing and temperature. Poor fibre coupling in the pack-age contributed to the low output power with an approximate coupling loss of 6dB. It is hopedthis will be improved to ≤ 3dB in future iterations of the device. It is worth noting here that thedevice is also suitable for monolithic integration with an SOA for increased output power.
3. Experiment
Figure 3 shows the experimental set up where the two sections of the device are labeled ‘Left’and ‘Right’. Firstly the device under test was characterized by varying the bias current to both
#176950 - $15.00 USD Received 28 Sep 2012; revised 15 Nov 2012; accepted 16 Nov 2012; published 30 Nov 2012(C) 2012 OSA 10 December 2012 / Vol. 20, No. 26 / OPTICS EXPRESS B401
sections as well as the operating temperature while observing the output wavelength, as de-scribed in section 2. Conditions which ensured single mode operation were noted and severalof these were used for data transmission.
OFDM pilots and transmission signals were created offline using Matlab. The Gain to NoiseRatio of each subcarrier was estimated by propagating OFDM pilot signals with 16 QuadratureAmplitude Modulation (QAM) on every subcarrier. The Levin-Campello (LC) bit/power load-ing algorithm [8] was then used to calculate the optimal bit distribution across those subcarrierswhich effectively bit/power loads by assigning different constellation sizes to each subcarrier.A 256 input Inverse Fast Fourier Transform (IFFT) was used and subcarrier spacing was setto 39.06MHz. The number of subcarriers, and hence OFDM bandwidth, used for each systemconfiguration was determined by the LC algorithm; typically between 3 and 4GHz. A CyclicPrefix (CP) of 6.25% was added and 7% of the AM-OFDM signal was reserved for ForwardError Correction (FEC). A real signal was created by modulating the real and imaginary com-ponents of the complex baseband AM-OFDM signal with the In-phase (I) and Quadrature (Q)components of an RF carrier respectively. The resultant signal was output from the Digital toAnalogue Converter (DAC) of an Arbitrary Waveform Generator (AWG) sampling at 10GSa/s.Typical Peak-to-Average Power Ratio (PAPR) of the AM-OFDM signals was 12dB.
The AM-OFDM signal was then used to directly modulate one section of the device in con-junction with a DC bias. The decision to modulate either the left or right side section dependedon which section could be modulated with the largest RF signal before the SMSR of the opticaloutput was reduced below 30dB. As stated, optical launch power varied depending on the oper-ating conditions. Transmission was carried out over 0km, 25km and 50km of Standard SingleMode Fibre (SSMF). Where necessary, the received optical signal was attenuated by a VariableOptical Attenuator (VOA) to an appropriate level so as to avoid saturation of the AvalanchePhoto-detector (APD) with an integrated Trans-impedance Amplifier (TIA); this occurred atapproximately -12dBm. The received RF signal was captured using a Real Time Oscilloscope(RTS) also sampling at 10GSa/s. Required Digital Signal Processing (DSP) including chan-nel estimation and equalization as well as Error Vector Magnitude (EVM) and Bit Error Rate(BER) calculation was completed offline.
4. Results and discussion
Fig. 4: Available modes spanning the range 1558nm to 1572nm. Insets show 16, 32and 64-QAM constellation diagrams on three modes selected for transmission.
Figure 4 shows superimposed optical spectra of some of the available modes attained usinga combination of temperature tuning and various biasing settings for each section. The insetsshow received 16, 32 and 64-QAM constellation diagrams on selected AM-OFDM subcarriersafter transmission over 25km on the 1560.02nm, 1564.46nm and 1571.72nm channels respec-tively. The device is shown to be tuneable from 1557.68nm to 1571.72nm giving a tuning rangeof 14.04nm. Work is currently being carried out to improve the continuous tuning capability ofthis device by examining performance over a wider range of temperatures.
#176950 - $15.00 USD Received 28 Sep 2012; revised 15 Nov 2012; accepted 16 Nov 2012; published 30 Nov 2012(C) 2012 OSA 10 December 2012 / Vol. 20, No. 26 / OPTICS EXPRESS B402
Table 1: Received raw data rates for all channels.
Seven modes were selected from across the range of available wavelengths for AM-OFDMtransmission to be performed. Table 1 shows the raw data rates achieved for each mode overall transmission distances while maintaining a BER of 1×10−3. Given that the optical launchpower varied due to differing biasing conditions, no optical amplification was used and thattransmission distance varied, the received optical power did not remain constant. This con-tributes to a decrease in data rates over 25km and 50km compared to the 0km case as shot andthermal noise at the receiver make a greater contribution to the Signal to Noise Ratio (SNR)of the lower received power AM-OFDM signals. Nevertheless, greater than 10Gb/s data ratesare displayed on all modes over 0km and 25km and on most modes over 50km. Data ratesmarked with an asterix in the 50km column fall short of the required raw data rate for 10Gb/stransmission given the CP and FEC overheads needed.
In addition to loss, the transmitted signals also experience dispersive fading over 50km. Thisis due to the double sideband nature of the transmitted signals and its effect is to introducenulls at frequencies which vary with transmission distance [9]. Subsequenty the GNRs of theaffected subcarriers are decreased, the bit distribution given by the LC algorithm is updated totake this into account and data throughput is decreased as is reflected in table 1. Figure 5 showsthe received electrical spectra of the same 16-QAM pilot signal (no power/bit loading) with74 subcarriers and centred at 1.6GHz over 25km (a) and 50km (b) on the 1564.46nm channelwith identical optical launch power. The effects of dispersive fading can clearly be seen as thereceived signal power decreases by up to 10dB at higher frequencies in the 50km case.
Fig. 5: Received electrical spectra over 25km (a) and 50km (b).
Figure 6 shows the bit and power loading distributions calculated using the rate adaptiveform of the LC algorithm for the 1569.92nm channel in the back-to-back case and over 25and 50km. The figure shows that the order of modulation decreases as subcarrier frequenciesenter the laser’s frequency range of non-linear operation. This occurs at frequencies close to theresonance peak of the modulation response and is due to to the non-linear interaction betweencarriers and photons in the laser cavity [10]. In the back-to-back case 120 subcarriers are usedso the OFDM bandwidth is given as ∼ 4.73GHz. By multiplying the bandwidth by the achieved
#176950 - $15.00 USD Received 28 Sep 2012; revised 15 Nov 2012; accepted 16 Nov 2012; published 30 Nov 2012(C) 2012 OSA 10 December 2012 / Vol. 20, No. 26 / OPTICS EXPRESS B403
Fig. 6: Bit and power (dashed) loading distributions used to generate the AM-OFDMsignal for use on the 1569.92nm channel, for all transmission distances.
spectral efficiency (average bits per symbol = 4.553) we can see that the data rate obtained hereis 21.52Gb/s. As stated, in the case of transmission over 25km the lower received optical powerleads to a disimprovement of SNR at the receiver. Subsequently the LC algorithm updates thebit distribution and some higher frequency subcarriers are dropped due to the new level ofpower required on those subcarriers to successfully transmit lower order formats. The powersaved by not transmitting these subcarriers is redistributed to maintain successful transmissionon other subacarriers and maximise throughput. The net effect is a decrease in throughput from21.52Gb/s to 16.64Gb/s. This effect is also evident over 50km where a further disimprovementof SNR coupled with the dispersive fading experienced over this transmission distance resultsin the number of subcarriers dropping to 74 and overall throughput dropping to 11.3Gb/s.
5. Conclusion
Direct modulation of a tuneable laser by an AM-OFDM signal has been shown for the firsttime to best of the authors’ knowledge. The tuneable dual section SFP laser under test hasbeen characterised in terms of SMSR and output wavelength for varying bias and temperatureconditions. Single mode operation can be achieved on over thirty modes across a spectral rangeof 14nm when using a combination of current and temperature tuning. Work is currently beingundertaken to increase both the number of available channels and the output power from thedevice therefore making it more suitable for OAN applications. Coolerless operation would befeasible for systems employing larger bandwidth filters.
AM-OFDM transmission is performed on seven selected modes and performance on eachis measured in terms of the maximum achievable data throughput. Data rates of greater than10Gb/s are displayed for all of these modes over 25km of SSMF and on the majority of modesover 50km where dispersive fading is the limiting factor on performance.
The application of direct modulation in conjunction with the highly spectrally efficient AM-OFDM signal and a tuneable slotted Fabry-Perot laser offers a potentially cost effective solu-tion for transmitters in OANs where low cost is of primary importance due to their high marketvolume. Moreover, the problems of insertion loss, polarisation dependence and footprint asso-ciated with systems employing external modulators are overcome, adding to the suitability ofthis system design for deployment in future OANs.
Acknowledgment
The authors acknowledge Achray Photonics for the high speed packaging of the SFP device.
#176950 - $15.00 USD Received 28 Sep 2012; revised 15 Nov 2012; accepted 16 Nov 2012; published 30 Nov 2012(C) 2012 OSA 10 December 2012 / Vol. 20, No. 26 / OPTICS EXPRESS B404
