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Next generation optical access networks will require low cost lasers in conjunction with network flexibility and higher data rates. This work presents the direct modulation of a low cost tuneable slotted Fabry-Pérot laser (tuneable over 14nm) with AM-OFDM. Characteristics of this dual section laser are presented and transmission of 10Gb/s over 50km is achieved with this device.
© 2012 Optical Society of America
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 by facilitating the use of identical Optical Networking Units (ONUs) within the network while still maintaining colourless operation [1]. However, for the advantages of tuneable lasers to be exploited in future PONs their complexity and associated cost need to be considerably reduced given the potentially wide deployment of these networks [2]. In order to reduce the cost and footprint of a tuneable transmitter in PONs, it is preferable to use direct modulation. This technique also avoids other problems associated with external modulators such as bias drift, insertion loss and polarization dependence. A tuneable slotted Fabry-Pérot (FP) laser is employed in this work. Compared to RSOAs the SFP eradicates the requirement for optical filtering at the ONU 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 already 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 each overlapping orthogonal subcarrier depending on the channels frequency response. This, coupled with its facilitation of a simple maximum likelihood equalizer in the frequency domain to overcome the effects of chromatic dispersion, makes AM-OFDM an excellent modulation technique 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 effective optical system. The device is first characterised so that conditions leading to single mode operation can be obtained. Then AM-OFDM is used to maximise the data throughput given the variable direct modulation bandwidth of the device at these different operating conditions.
2. Laser device
The laser device used in this work was a dual section SFP laser diode. The slots are etched into the top of an otherwise conventional laser ridge waveguide transforming the multimode spectrum of the FP laser into a very high quality single mode device [5]. Increased tuning range operation of the laser is achieved by optimising the laser’s mirror reflectivities in each section by employing the Vernier effect to extend the tuning range associated with the limited refractive index change with current [6]. This device requires no additional re-growth or processing steps when compared with other tuneable devices, which minimises the fabrication complexity. The device is controlled by a Thermo-Electric Cooler (TEC) and both of its sections may be biased independently. Wavelength tuning is achieved as single mode operation, defined as Side Mode Suppression Ratio (SMSR) ≥ 30dB [7], can be attained at various wavelengths depending upon the biasing of both sections and temperature conditions. To ascertain bias conditions for which single mode lasing is achieved, SMSR and output wavelength were measured for a set of bias currents to each section ranging from 0 to 80mA in 1mA steps. This process was repeated for various temperatures ranging from 2.5°C to 17°C. Figure 1 shows such an SMSR map obtained at 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 conditions do not result in single mode operation. Work is ongoing to increase the number of available modes of this prototype device, including those compatible with ITU grid wavelengths, while minimising cost so as to maintain its suitability for access networks. Approaches to achieve this include further optimisation of the etched features in order to increase the reflection spectrum from the slots and the inclusion of passive sections to allow a wider tuning range to be achieved due to the larger refractive index changes with current in these regions.
Fig. 1: An SMSR map of the tuneable device. The graduated scale gives the SMSR so that biasing conditions which result in single mode lasing can be clearly identified.
For each set of single mode lasing conditions there was an associated modulation response for each section. Figure 2 shows modulation responses for three sets of these conditions and the modulation bandwidths achieved are around 4GHz. The output power of the device varied between −3 and −7dBm depending on biasing and temperature. Poor fibre coupling in the package contributed to the low output power with an approximate coupling loss of 6dB. It is hoped this will be improved to ≤ 3dB in future iterations of the device. It is worth noting here that the device 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 sections as well as the operating temperature while observing the output wavelength, as described in section 2. Conditions which ensured single mode operation were noted and several of these were used for data transmission.
OFDM pilots and transmission signals were created offline using Matlab. The Gain to Noise Ratio of each subcarrier was estimated by propagating OFDM pilot signals with 16 Quadrature Amplitude Modulation (QAM) on every subcarrier. The Levin-Campello (LC) bit/power loading algorithm [8] was then used to calculate the optimal bit distribution across those subcarriers which 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 set to 39.06MHz. The number of subcarriers, and hence OFDM bandwidth, used for each system configuration was determined by the LC algorithm; typically between 3 and 4GHz. A Cyclic Prefix (CP) of 6.25% was added and 7% of the AM-OFDM signal was reserved for Forward Error Correction (FEC). A real signal was created by modulating the real and imaginary components 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 to Analogue 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 conjunction with a DC bias. The decision to modulate either the left or right side section depended on which section could be modulated with the largest RF signal before the SMSR of the optical output was reduced below 30dB. As stated, optical launch power varied depending on the operating conditions. Transmission was carried out over 0km, 25km and 50km of Standard Single Mode Fibre (SSMF). Where necessary, the received optical signal was attenuated by a Variable Optical Attenuator (VOA) to an appropriate level so as to avoid saturation of the Avalanche Photo-detector (APD) with an integrated Trans-impedance Amplifier (TIA); this occurred at approximately −12dBm. The received RF signal was captured using a Real Time Oscilloscope (RTS) also sampling at 10GSa/s. Required Digital Signal Processing (DSP) including channel estimation and equalization as well as Error Vector Magnitude (EVM) and Bit Error Rate (BER) calculation was completed offline.
4. Results and discussion
Figure 4 shows superimposed optical spectra of some of the available modes attained using a combination of temperature tuning and various biasing settings for each section. The insets show received 16, 32 and 64-QAM constellation diagrams on selected AM-OFDM subcarriers after transmission over 25km on the 1560.02nm, 1564.46nm and 1571.72nm channels respectively. The device is shown to be tuneable from 1557.68nm to 1571.72nm giving a tuning range of 14.04nm. Work is currently being carried out to improve the continuous tuning capability of this device by examining performance over a wider range of temperatures.
Fig. 4: Available modes spanning the range 1558nm to 1572nm. Insets show 16, 32 and 64-QAM constellation diagrams on three modes selected for transmission.
Seven modes were selected from across the range of available wavelengths for AM-OFDM transmission to be performed. Table 1 shows the raw data rates achieved for each mode over all transmission distances while maintaining a BER of 1 × 10−3. Given that the optical launch power varied due to differing biasing conditions, no optical amplification was used and that transmission distance varied, the received optical power did not remain constant. This contributes to a decrease in data rates over 25km and 50km compared to the 0km case as shot and thermal 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 rates are displayed on all modes over 0km and 25km and on most modes over 50km. Data rates marked with an asterix in the 50km column fall short of the required raw data rate for 10Gb/s transmission given the CP and FEC overheads needed.
Table 1:. Received raw data rates for all channels.
In addition to loss, the transmitted signals also experience dispersive fading over 50km. This is due to the double sideband nature of the transmitted signals and its effect is to introduce nulls at frequencies which vary with transmission distance [9]. Subsequenty the GNRs of the affected subcarriers are decreased, the bit distribution given by the LC algorithm is updated to take this into account and data throughput is decreased as is reflected in table 1. Figure 5 shows the received electrical spectra of the same 16-QAM pilot signal (no power/bit loading) with 74 subcarriers and centred at 1.6GHz over 25km (a) and 50km (b) on the 1564.46nm channel with identical optical launch power. The effects of dispersive fading can clearly be seen as the received signal power decreases by up to 10dB at higher frequencies in the 50km case.
Figure 6 shows the bit and power loading distributions calculated using the rate adaptive form of the LC algorithm for the 1569.92nm channel in the back-to-back case and over 25 and 50km. The figure shows that the order of modulation decreases as subcarrier frequencies enter the laser’s frequency range of non-linear operation. This occurs at frequencies close to the resonance peak of the modulation response and is due to to the non-linear interaction between carriers and photons in the laser cavity [10]. In the back-to-back case 120 subcarriers are used so the OFDM bandwidth is given as ∼ 4.73GHz. By multiplying the bandwidth by the achieved spectral efficiency (average bits per symbol = 4.553) we can see that the data rate obtained here is 21.52Gb/s. As stated, in the case of transmission over 25km the lower received optical power leads to a disimprovement of SNR at the receiver. Subsequently the LC algorithm updates the bit distribution and some higher frequency subcarriers are dropped due to the new level of power required on those subcarriers to successfully transmit lower order formats. The power saved by not transmitting these subcarriers is redistributed to maintain successful transmission on other subacarriers and maximise throughput. The net effect is a decrease in throughput from 21.52Gb/s to 16.64Gb/s. This effect is also evident over 50km where a further disimprovement of SNR coupled with the dispersive fading experienced over this transmission distance results in the number of subcarriers dropping to 74 and overall throughput dropping to 11.3Gb/s.
Fig. 6: Bit and power (dashed) loading distributions used to generate the AM-OFDM signal for use on the 1569.92nm channel, for all transmission distances.
5. Conclusion
Direct modulation of a tuneable laser by an AM-OFDM signal has been shown for the first time to best of the authors’ knowledge. The tuneable dual section SFP laser under test has been characterised in terms of SMSR and output wavelength for varying bias and temperature conditions. Single mode operation can be achieved on over thirty modes across a spectral range of 14nm when using a combination of current and temperature tuning. Work is currently being undertaken to increase both the number of available channels and the output power from the device therefore making it more suitable for OAN applications. Coolerless operation would be feasible for systems employing larger bandwidth filters.
AM-OFDM transmission is performed on seven selected modes and performance on each is measured in terms of the maximum achievable data throughput. Data rates of greater than 10Gb/s are displayed for all of these modes over 25km of SSMF and on the majority of modes over 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-Pérot laser offers a potentially cost effective solution for transmitters in OANs where low cost is of primary importance due to their high market volume. Moreover, the problems of insertion loss, polarisation dependence and footprint associated with systems employing external modulators are overcome, adding to the suitability of this system design for deployment in future OANs.
Acknowledgment
The authors acknowledge Achray Photonics for the high speed packaging of the SFP device.
References and links
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