Optical Frequency Comb synthesis for super channel based high-bandwidth data communication

Increasing demand for high bandwidth data communication requires development of innovative, reliable and scalable systems. Super-channels with Higher Order Modulation schemes will drive the next generation optical communication systems. The present work focusses on optical sources for such systems. Conventional implementations require hundreds of lasers and are not scalable; the proposed alternative is to develop and use an Optical Frequency Comb (a series of evenly spaced spectral components). Generating such a comb with precise spacing and stability is a challenge. This is addressed by system design using Electro-Optic Modulators and three lasers where we demonstrate a 24-line comb which is de-multiplexed to realize optical source for super channels. This is a scale up of number of carriers by a factor of 8.


Introduction
Increasing demand for high bandwidth data communication requires development of innovative reliable and scalable systems. According to Cisco's analysis and forecast [1], by 2020, the global IP (Internet Protocol) traffic will reach * 600 Tbps (194.4 Exa Bytes per month) ( Fig. 1) a near three-time increase of the IP traffic in 2015. The increase in traffic demand is from various developments such as Internet of Things (IoT), data backup over cloud, online gaming, home automation, immersive video, live feed of surveillance, and from increasing use of high definition videos [1].
Traditionally, the band width demand was met by parallelism in wavelength domain called as Wavelength Division Multiplexing (WDM) (Fig. 2) which involved the use of several laser sources for generating the optical carriers. This is bulky and expensive because of the large number of lasers required and their control circuits (Temperature and Current Control). Moreover, the drifts in different lasers are independent of each other necessitating drift corrections to the lasers to be applied individually. Spectral efficiency, defined as the ratio of data rate to allocated band width is an important metric for a transmission scheme. In WDM, the spectrum is inefficiently utilized as guard bands are necessary between adjacent channels to mitigate cross talk.
A major limitation in WDM implementation is the maximum achievable baud rate per channel which is limited by the electronics. Currently, the best electro-optic modulators operate only at 40 GBaud (Lithium Niobate based) making it the upper limit on realizable baud rate per channel. This electronic bottle neck can be overcome by the use of Super Channels where each channel is composed of several orthogonal sub-carriers [2]. The orthogonality condition requires that the frequency spacing between the sub-carriers be made precisely equal to the baud rate [2]. All the sub-carriers act as a single data channel (aka the Super Channel) and each of the sub-carriers can be modulated with the fastest modulators thus increasing the per channel capacity. Further, the existing baud rates can be achieved with low band width modulators which is more feasible (Fig. 3). The various sub-carriers in a super channel are routed together over the optical link to the same destination. A major requirement of these sub-carriers is that they must be frequency locked to each other so that the sub-carrier spacing does not vary. This is essential in order to satisfy the condition of orthogonality.
From Fig. 3b, it can be seen that there is significant overlap between adjacent sub-carriers in spite of which the data can be recovered because of the orthogonality of subcarriers. As there is no guard band between adjacent subcarriers this leads to efficient utilization of spectrum (High spectral efficiency). This technique is similar to the Orthogonal Frequency Division Multiplexing (OFDM) used in Wireless Networks and is hence called Optical OFDM (Fig. 4) [2].
From Fig. 4, it can be seen that the heart of this architecture is the multi-carrier generator which produces frequency locked orthogonal sub-carriers. The present work constitutes the implementation of such a multi-carrier generator with precise spacing and stability. We propose to use an Optical Frequency Comb as the source of orthogonal sub-carriers in a super channel. An Optical Frequency Comb (OFC) is a series of evenly spaced discrete spectral components with high spectral coherence across the entire bandwidth [3]. Thus the proposed scheme would overcome the electronic bottle neck and provides scalability as several carriers are produced from few lasers, thus overcoming all the drawbacks of WDM based system. Generating such a comb with precise spacing and stability is a challenge.
Optical frequency combs revolutionized optical synthesis and metrology, covering a broad range of applications, from molecular spectroscopy to laser ranging and optical communications [3]. One half of Nobel Prize for physics in 2005 was awarded to John L. Hall and Theodor W. Hänsch for their work involving the use of optical frequency combs in laser-based precision spectroscopy.
An Optical Frequency Comb (Fig. 5) is characterized by its central frequency f 0 and repetition rate f rep which is the   frequency spacing between adjacent lines. It is desired to have tunable central frequency and repetition rate so that the same system can cater to a wide range of applications. Traditionally, mode locked lasers were used to generate Optical Frequency Combs, but they suffer from lack of stability at high repetition rates and low tunability [4].
Electro Optic Modulators can be used to generate an Optical Frequency Comb with stability at high repetition rates [4][5][6][7][8][9] along with the added benefits of tunability of central frequency and repetition rate.
An Electro Optic Modulator consists of a material whose refractive index varies with the applied drive voltage. A time varying drive voltage (here RF source), produces a time varying refractive index which leads to phase modulation of the optical signal travelling through the material. The phase modulation of the optical carriers produces side bands which are the teeth of the optical frequency comb.
The central frequency of the frequency comb can be tuned easily by tuning the laser. Here, the repetition rate of the comb is equal to the RF drive frequency and thus the repetition rate can be tuned by changing the RF drive frequency.

Experimental setup and results
The schematic of Electro-Optic Modulator based Optical Frequency Comb generator is shown in Fig. 6.
A 1550 nm source is used as optical input to the cascaded phase modulator system. The Electro Optic   Modulator power handling capacity limits the strength of modulation (and thus the number of comb lines generated) which is overcome by cascaded Phase Modulators. Figure 7 shows the generated optical frequency comb with 11 lines within a 30 dB range from the peak power. The system is scaled up by increasing the number of input laser wavelengths to two and three. Figure 8a shows the output frequency comb from two lasers. The power levels vary widely among the comb lines. As these comb lines are to be used as optical carriers, a uniform signal to noise ratio is desired to have identical performance in all the carriers. To achieve this, a programmable optical filter is used and the equalized 16-line comb is shown in Fig. 8b. A Booster Optical Amplifier (BOA) is used post the filter to increase the output power. Figure 9a shows the optical frequency comb generated from three laser sources and Fig. 9b shows the equalized 24-line comb. The flatness of the comb is better than 1.6 dB (variation in power). The number of lasers are increased by a factor of 8 and a total bandwidth of 600 GHz is demonstrated. The theoretically achievable data rate when these carriers are modulated with the advanced modulation formats of Polarization Multiplexed Each of the three super channels constitutes 8 sub-carriers encompassing 200 GHz. The per channel baud rate has been increased by a factor of 4 to 200 GBd. To modulate data onto each of the carriers, they need to be separated from the comb. A combination of De-Interleaver and De-Multiplexers is used to achieve this. The De-interleaver is a step phase Michelson interferometer [10] based device which separates alternate lines into the even and odd output channels (Fig. 10). This converts a 25 GHz spaced optical   frequency comb into two sets of 50 GHz spaced optical frequency combs. The obtained spectra at even and odd channels is shown in Fig. 11.
The outputs of even and odd channels are passed through two Arrayed Waveguide (AWG) based de-multiplexers to separate out the individual carriers (Fig. 12). One of the De-multiplexers is aligned to ITU-50 GHz grid and the other is offset by 0.2 nm (25 GHz). Figure 13 shows the eight de-multiplexed sub-carriers of one of the generated super channels.
Each super channel can be sub-divided as desired. One such division to implement 100 GHz super channels is shown in Fig. 14.

Conclusions and future work
An Optical Frequency comb of 24 lines with 25 GHz repetition rate was successfully generated from three laser seeds using cascaded EOMs. This is a scale up of number of carriers by a factor of 8.
Three Super Channels, each with a bandwidth of 200 GHz are implemented. A total bandwidth of 600 GHz is demonstrated which is capable of realizing data rates of 2.4 and 4.8 Tbps with PM-QPSK and PM-16 QAM modulation schemes respectively.
De-Multiplexing is done to modulate data on to individual sub-carriers for high bandwidth communication system implementation.
Future work would involve further bandwidth scaling, modulation and demodulation on the combs to generate fully operational [ 1 Tbps SC.