The authors present the performance and noise properties of a software reconfigurable, FSR and wavelength tunable gain switched optical frequency comb source. This source, based on the external injection of a temperature tuned Fabry-Pérot laser diode, offers quasi-continuous wavelength tunability over the C-band (30nm) and FSR tunability ranging from 6 to 14GHz. The results achieved demonstrate the excellent spectral quality of the comb tones (RIN ~-130dB/Hz and low phase noise of 300kHz) and its outstanding stability (with fluctuations of the individual comb tones of less than 0.5dB in power and 5pm in wavelength, characterized over 24hours) highlighting its suitability for employment in next generation flexible optical transmission networks.
© 2015 Optical Society of America
Flexible optical networks, also referred to as elastic networks , are gaining a lot of attention thanks to their potential to provide higher data rate, enhanced spectral efficiency and improved flexibility to meet the incessant exponential growth of global data traffic . Flexible networks enable an efficient usage of the available spectrum by dynamically allocating the bandwidth (elastic spectrum allocation) [3, 4] according to the traffic demands. Moreover, an optimized spectral efficiency and performance is offered by adjusting transmission parameters such as sub-channel bandwidths, information rates and modulation formats depending on the unoccupied bandwidth, required capacity and optical reach. These networks require the spectral grid to evolve towards finer granularities or operate on a gridless architecture [5, 6] that allows the combination of an arbitrary number of small frequency slots to create bandwidth-fitted superchannels. This maximizes the use of the available spectrum and enables the transmission of high bit rate superchannels (400 Gb/s, 1Tb/s, etc.) [7, 8] without increasing the order of the modulation format, which would impact the maximum transmission distance . Thus, new standards such as the Flexible grid defined by the ITU-T in the G.694.1 recommendation  are emerging to support their deployment.
To support the aforementioned developments, flexible networks require innovative components such as spectrum selective switches (SSSs) and flexible transponders for switching, transmission and reception . A key component in the transponders of these flexible optical systems may be an optical frequency comb (OFC) source [11, 12], which enables the reduction or elimination of guard bands by ensuring constant frequency spacing between the carriers. Particularly, flexible networks would certainly benefit from an OFC that offers free spectral range (FSR) and wavelength tunability. These capabilities allow a single source to be easily adapted to suit the chosen symbol rate and to allocate a superchannel simultaneously to a specific wavelength band, thus reducing operational costs.
Previously, we reported on the generation of a wavelength tunable OFC by gain switching a commercially available Fabry-Pérot (FP) laser and externally injecting into a particular longitudinal mode . External injection offers beneficial properties such as an improvement in the number of comb tones, spectral flatness and an effective transference of the narrow linewidth of the master laser to the slave comb lines, making such comb sources particularly attractive for coherent superchannel applications . However, the wavelength tunability demonstrated in  is discrete and localized at those wavelengths where the FP laser exhibits a longitudinal mode. Moreover, these OFCs are far from network deployment as they require an accurate manual re-adjustment and optimization of the injection parameters (injected power and wavelength detuning) to successfully generate a flat comb when varying the FSR or operating wavelength [13, 15]. Hence, these factors are potentially limiting the flexibility of the transmitter and its reconfiguration, thus, making it less attractive in the emerging flexible grid networks.
In this work, we present and describe a highly flexible OFC source that can be software reconfigured to offer FSR and wavelength tunability. These features enable automatic setting of the parameters to obtain an optimum comb (in terms of flatness and noise properties) targeting various flexible network applications. As a result, a highly coherent optical comb source consisting of 6 clearly resolved 12.5 GHz tones delivering ~2 dBm output power per comb line with an extinction ratio in excess of 50 dB is achieved. The FSR can be software reconfigured from 6 to 14 GHz while the wavelength can be quasi-continuously tuned along the whole C-band (1535-1565 nm = 30 nm). We also present a stability characterization of the externally injected gain switched OFC source and demonstrate its robustness over 24 hours. Results achieved show that the highest fluctuation of the individual comb tones in power is about 0.5 dB and in wavelength is approximately 5 pm. Moreover, we characterize the phase noise properties of the comb source by carrying out a detailed analysis of the FM-noise spectrum. This analysis can provide a more thorough information and understanding of the different noise processes contributing to the overall phase noise, as opposed to the 3 dB optical linewidth which might not be sufficient [13, 16]. Experimental results highlight the excellent spectral quality of the comb tones and performance consistency, with relative intensity noise (RIN) ~-130 dB/Hz and low phase noise (estimated linewidth 300 kHz), at any operating wavelength across the C-band.
To the best of our knowledge, this is the first time a stand-alone software reconfigurable comb source, highly flexible in FSR and wavelength has been reported, and fully characterized in terms of long term stability and FM-noise property. To further enhance the practical potential and viability of this reconfigurable comb source, photonic integration  and comb line expansion  techniques could be also implemented.
The article is organized as follows. In section 2, the software reconfigurable gain switched OFC is described in detail. In section 3, the reconfigurability of the OFC is analyzed starting with the flexible FSR and subsequently the operating wavelength tunability. Section 4 comprises a long term stability measurement in terms of power and wavelength variation of the individual comb tones and section 5 presents the characterization of the intensity and phase noise properties of the source. Finally our conclusions are presented in section 6.
2. Reconfigurable comb source
The reconfigurable OFC source is presented in Fig. 1. The source comprises a commercially available FP laser which is encased in an optically un-isolated and temperature controlled high-speed butterfly package with an internal bias tee. The laser exhibited a threshold current (Ith) of 8 mA at room temperature and a small signal modulation bandwidth of 11 GHz when biased at 40 mA (5Ith) as illustrated in the inset (a) of Fig. 1. A semiconductor based tunable laser (TL) with low linewidth (~300 kHz), acting as a master, injects light into the cavity of the FP laser via a polarization-maintaining (PM) circulator. PM fibers are used, throughout the OFC, to maintain the state of polarization thereby enabling automated control of reconfigurable features. Their use removes the need of polarization controllers to align the polarization state of the injected light with the optical waveguide of the FP laser. This alignment is subjected to environmental variations such as temperature, mechanical vibrations, pressure, etc. which could cause instabilities. The wavelength and power of the TL are carefully chosen to ensure injection locking on a particular longitudinal mode of the FP laser, thereby the other modes are suppressed and single mode operation of the FP laser is achieved at the selected wavelength. An RF (K) connector attached to the FP laser package enables gain switching by direct modulation with an amplified sinusoidal RF signal (~24 dBm) in conjunction with a dc bias of 45 mA, resulting in a frequency comb generation around the injection locked single longitudinal mode. The large RF signal is achieved by using an RF amplifier driven with a sinusoidal signal from an external synthesizer.
Injection locking into the different longitudinal modes of the FP laser can be obtained by modifying the wavelength of the TL, thereby, generating a wavelength tunable OFC within the C-band . In order to achieve greater flexibility with quasi-continuous tunability and not only localized to a set of discrete wavelengths, the FP laser temperature could be also varied accordingly using the internal TEC in the butterfly module which would induce a wavelength shift in its longitudinal modes. Accurate and fine tuning of the wavelength is required for network deployment and could prove complex, therefore we automate this process with the aid of a microcontroller and a software program to control the parameters of the reconfigurable OFC without requiring manual optimization. The parameter setting algorithm calculates, according to the linear dependence of the wavelength with the temperature of Δλ/ΔT of 0.1 nm/°C  and the Steinhart-Hart equations , the thermal tuning of the FP laser needed to obtain a longitudinal mode at the wavelength required. Then, the microcontroller (µc) controls a 12 bit digital-to-analog converter (DAC) that varies the temperature through an analog voltage connected to the temperature and current controller. Finally, the software program communicates with the TL to set the required detuning and injected power to accomplish the optimum injection locking on that longitudinal mode.
In this experimental set up the software program also controls a synthesizer to set the FSR. The resultant comb is then observed with a high resolution (0.16 pm) optical spectrum analyzer (OSA). The footprint of the reconfigurable OFC could be potentially reduced by including an internal synthesizer (or alternatively, a voltage controlled oscillator). Additionally, this source lends itself to photonic integration , thereby enhancing its compactness, robustness and ease of manufacture.
In this section, the aspects of OFC reconfigurability are analyzed in detail starting with the flexible FSR and subsequently the widely and quasi-continuous tunable operating wavelength.
3.1 FSR tunability
Figures 2(a)-2(e) illustrates the OFC generated with repetition rates spanning a range of 6.25-14 GHz by simply changing the RF synthesizer frequency. Gain switching at these FSRs results in the generation of highly coherent [13, 17] frequency combs consisting of 4-14 clearly resolved comb tones within 3 dB of the spectral envelope peak with an extinction ratio of around 50 dB and a power per line of ~2 dBm. This variation in the number of comb tones at different FSRs is a restraint dictated by the inherent frequency response of the slave laser, illustrated in the inset (a) of Fig. 1, and the linewidth enhancement factor. Hence, a smaller FSR (lower frequency) will yield a larger number of tones (better modulation response at low frequencies). Other comb generation configurations using cascaded modulators and highly nonlinear fibre (HNLF) [20, 21] have reported wider spectral combs. However, these techniques suffer from a higher complexity, footprint, instability and power consumption. It should be noted that the source presented here could also avail from expansion techniques using phase modulators , HNLF  or by cascading several gain switched FP lasers  which is a more cost-efficient and flexible alternative to enhance the practicality and potential for network deployment.
The 6.25 GHz and 12.5 GHz FSRs with 14 and 6 comb lines within a 3 dB flatness window respectively are given substantial attention here due to their direct compatibility with the proposed ITU-T flexible grid recommendation in G.694.1 . Furthermore, the high degree of flexibility will allow the use of this comb source in future flexible networks where the frequency slot might be further reduced [5, 6]. It is important to note that even though this paper focusses on a narrow band of FSR tunability, the technique offers itself for continuous broadband FSR tunability ranging from 5 to 40 GHz .
3.2 Quasi-continuous wavelength tunability across the whole C-band
We then demonstrate the wavelength tunability of this comb source over the C-band. For this purpose, the TL is tuned in wavelength to inject −3 dBm into different longitudinal modes of the FP laser. This precise injection results in a single mode operation that is subsequently gain switched with an amplified 10 GHz sinewave to generate the comb. Figures 3(a)-3(c) present the resultant reconfigurable gain switched comb source, with ~7 clearly resolved comb lines within a 3 dB spectral envelope and more than 50 dB of extinction ratio, at central wavelengths of 1535 nm, 1550 and at 1565 nm, respectively.
Precise wavelength tunability between the longitudinal modes can be achieved by fine thermal tuning of the FP laser. The wavelength drift of the longitudinal modes due to temperature variation is typically dictated by the linear relation Δλ/ΔT of 0.1nm/°C. The temperature of the FP laser is controlled by interrogating a thermistor inside the package, which serves as an inexpensive and accurate temperature monitor . The nonlinear resistance versus temperature (value of the thermistor resistance for a certain temperature) characteristics of a thermistor may be modeled to a high degree of accuracy using the Steinhart-Hart equation:
where T is the temperature of the laser (in Kelvins), R (in Ohms) is the resistance at a temperature T, and A, B and C are the Steinhart-Hart coefficients which vary depending on the type and nominal value of the thermistor. In our experiment, the nominal value of the thermistor in the package of the FP was 10 kΩ and therefore, the Steinhart-Hart coefficients are :
In order to obtain the set temperature of the FP laser required, given the temperature tuning required to produce the wavelength drift of the longitudinal modes, the inverse Steinhart-Hart equation is used:
Then, the wavelength of the TL is detuned accordingly to ensure injection locking and single mode operation. In Fig. 4 the wavelength of the 10 GHz spaced comb (with 7 clearly resolved comb tones in a 3 dB window) is being tuned by 0.48 nm at a time (changes in the FP temperature of 4.8 °C) covering a 5 nm span as a proof-of-concept demonstration.
The wavelength tunability is quasi-continuous with a minimum wavelength step of 1 pm (~125 MHz) limited by the wavelength setting resolution of the TL being used.
The coarse central wavelength tuning time (between any longitudinal modes) across the C-band would be determined by the switching speed of the TL used, which could typically be in the nanoseconds range.
Finally, the quasi-continuous wavelength tuning time is determined by the settling time of the slave laser to a new temperature, which is in the milliseconds range. Therefore, the thermal tuning would be used for fine wavelength tuning and could be employed for applications in flexible networks where millisecond-level reconfiguration times are suitable, such as the allocation of new connections to the network, re-allocation of the spectrum to avoid fragmentation, management or restoration schemes [1, 24].
OFCs generated by strongly driven electro-optic modulators suffer from bias drifts over long periods of time  and may require a feedback based dc bias control to maintain stable operation . In comparison, externally injected gain switched OFCs have been presented as a more stable alternative. However, this superior stability has never been fully characterized in the literature.
The two main parameters governing the overall stability of an OFC are the optical power and wavelength variations of individual comb tones over time. Hence, these two parameters are measured in the externally injected gain switched tunable OFC source by monitoring the optical spectrum of the comb source at ambient temperature over a stabilized bench with a high resolution OSA (0.16 pm). Traces were captured every 30 seconds over a total period of 24 hours. The comb monitored was generated with an FSR of 12.5 GHz and at a central wavelength of 1550.5 nm. For that, the temperature of the FP laser was tuned to 25°C and the tunable laser was injection locking with an optical power of −3 dBm into a particular longitudinal mode of the FP laser. Figure 5 illustrates the power fluctuations over time of the 6 clearly resolved comb lines within a spectral ripple of 3 dB, depicted in the Fig. 2(b). The maximum fluctuation in optical power over 24 hours was found to be 0.5 dB for the worst case (limited by a power resolution of 0.2 dB of the OSA in use). It can be observed that not all the comb tones present the same level of power variation. The smallest deviation is only 0.2 dB for the strongest comb tone in power. In comparison to electro-optic modulators based comb generation techniques that have shown up to 1.9 dB power fluctuations in a three-hour measurement , this small deviation proves that comb generation by gain switching an externally injected laser is a simple technique that offers excellent stability suitable for network deployment without any additional controllers.
Figure 6 shows a simultaneous measurement carried out where the wavelengths of the same individual comb tones were monitored. In this case, the wavelength deviation of any comb line is no more than 5 pm. It has to be mentioned that the wavelength deviation for each comb tone is the same as the frequency spacing between the comb tones is fixed. Thus, all comb tones experience the same variation and drift together.
As such, these measurements highlight the robustness of the proposed reconfigurable OFC source which offers long-term operational stability without requiring automated feedback control or manual adjustments.
5. Noise properties
The final aspect of the characterization includes RIN and phase noise measurements on the reconfigurable comb source, as these parameters will determine the system performance that can be achieved when employing these comb sources in communication systems employing advanced modulation format transmission.
5.1 Relative intensity noise (RIN)
The RIN is an important parameter to characterize as it is an indicator of the noise properties of the transmitter . For clarity of demonstration, RIN measurements were carried out at three selected operating wavelengths (1535, 1550 and 1565 nm). For each operating wavelength the RIN of three individually filtered comb lines (indicated by arrows in Figs. 3(a)-3(c) at the left edge, central and right edge of the 3 dB window) and the RIN of the overall unfiltered comb are measured. They are then compared to the RIN of the TL (master) free running.
The achieved results are shown in Figs. 7(a)-7(c). In these figures, the noise floor presented as a reference (−140 dB/Hz) is the minimum perceivable noise level of the equipment used in these measurements. The averaged RIN (DC to 10 GHz) is −129 dB/Hz for all the scenarios mentioned, which is the same as the RIN of the TL. The low RIN of the comb tones (comparable to the RIN of the TL) is a beneficial outcome of the external injection which reduces the overall RIN of the slave laser and thus, the RIN associated with each of the comb tones . Moreover, these results demonstrate that this type of gain switched OFC is not affected by mode partition noise unlike mode-locked laser based comb sources . It is important to note that the RIN measured is consistent for the three different operating wavelengths, hence, similar system performance should be expected across the C-band (30 nm).
5.2 Phase noise
The phase noise of an optical transmitter is a crucial parameter when advanced modulation formats are imposed for coherent superchannel applications . Previous studies have pointed out that optical linewidth measurement techniques can significantly broaden the observed optical linewidth due to excess 1/f frequency noise [29, 30]. Thus, a detailed analysis of the FM-spectrum is required to fully describe the noise processes that affect the phase noise properties as well as their origin [16, 29].
In this reconfigurable gain switched OFC, the observed optical linewidth could potentially be affected by the low frequency noise from the electronics (switchable power supply, current sources, microcontroller, etc.) that transfers to the lasers. We thus need to characterize the FM-noise spectrum as the intrinsic optical linewidth can be obtained from the white component of the frequency noise alone. It is worth mentioning that the presence of 1/f noise would not affect the performance of coherent superchannel applications as it will result in slow frequency fluctuations .
The phase noise measurements were carried out on the same comb lines and wavelengths previously mentioned in the RIN measurement, marked in Figs. 3(a)-3(c). These measurements are then compared with the phase noise characteristic of the TL (master) at those wavelengths over a frequency range up to 1 GHz, limited by the digital filtering in the offline processing [16, 29]. As illustrated in Figs. 8(a)-8(c), the FM-noise spectrum of the master is dominated by white FM noise (corresponding to an intrinsic linewidth of 300 kHz) and a small high-frequency component. The master FM-noise does not present a 1/f noise component at the frequencies measured using our set up (which measures down to 2 MHz), thus, the observed optical linewidth would not be affected. The filtered comb lines at each wavelength have identical FM-noise spectrum as the master at lower frequencies (below 200 MHz). Therefore, the optical linewidth of the comb tones follows that of the master [13, 16].
At high frequencies the phase noise of the filtered comb lines presents a minimum deviation from the master. This deviation was studied in  and related to the injection parameters used (injected power and wavelength detuning). From the results presented in Figs. 8(a)-8(c), the deviation in the FM-noise spectrum is minimized hence suggesting optimum injection parameters for the reconfigurable OFC. These results indicate that the comb tones would impose zero or minimum penalties (compared to the master) when being employed in a coherent communication system . The potential penalty, if there is any, will depend on the detailed system parameters (modulation format, symbol rate, etc.) and is a subject for further investigation.
Finally, we point out that the three operating wavelengths present a similar FM-noise spectrum. It could be expected that as we inject into the extreme longitudinal modes in the gain curve of the FP laser (1535 nm and 1565 nm), the injection locking could be more challenging to achieve than in the center of the FP gain curve (1552 nm), where the longitudinal modes have more power concentrated as illustrated in the inset (b) of Fig. 1. These results highlight that proper chosen injection parameters (injected power and detuning) can guarantee consistent performance and phase noise properties across the C-band.
We have presented a highly flexible software reconfigurable optical comb source and provided detailed characterization of all of the major parameters of interest for the flexible optical networks. The software which allows the re-configurability and selection of the comb parameters could be applied to an elastic optical network for dynamic spectrum allocation and adjustment of transmission parameters. The source offers wide FSR and wavelength tunability over the C-band, generated by seeding a thermally tuned gain switched FP laser with a tunable laser. These flexible features, in conjunction with a demonstrated exceptional long term stability and consistent low RIN and low phase noise performance for any operating wavelength, prove the suitability of this source for next generation flexible optical transmission networks. Moreover, the externally injected gain switched scheme intrinsically offers key parameters for network deployment such as simplicity, cost-efficiency and potential for monolithic integration that would further reduce the footprint and power consumption of the source.
The research leading to these results has received funding from the Irish Research Council employment based postgraduate scheme EBP-2012, the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 619591, the Science Foundation Ireland (grants 14/TIDA/2405 and 12/RC/2276), Enterprise Ireland (grant CF/2013/ 3617) and HEA PRTLI 4 INSPIRE Programs.
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