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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 3 — Feb. 11, 2013
  • pp: 3342–3353
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Scalable modulator for frequency shift keying in free space optical communications

Shelby Jay Savage, Bryan S. Robinson, David O. Caplan, John J. Carney, Don M. Boroson, Farhad Hakimi, Scott A. Hamilton, John D. Moores, and Marius A. Albota  »View Author Affiliations


Optics Express, Vol. 21, Issue 3, pp. 3342-3353 (2013)
http://dx.doi.org/10.1364/OE.21.003342


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Abstract

Frequency shift keyed (FSK) modulation formats are well-suited to deep space links and other high loss links. FSK’s advantage comes from its use of bandwidth expansion. I.e., FSK counteracts power losses in the link by using an optical bandwidth that is greater than the data rate, just as pulse position modulation (PPM) does. Unlike PPM, increasing FSK’s bandwidth expansion does not require increased bandwidth in electronic components. We present an FSK modulator whose component count rises logarithmically with the bandwidth expansion. We tested it with four-fold bandwidth expansion at 5 and 20 Gbit/s. When paired with a pre-amplified receiver, the required received power was about 4 and 5 dB from the theoretical best for such receivers. We also tested the FSK transmitter with a photon counting receiver.

© 2013 OSA

1. Introduction and background

Another advantage comes from the large bandwidths available in optical components and channels – 4 THz in the gain region of erbium-doped fiber amplifiers (EDFAs). We can use this bandwidth to obtain higher data rates and to improve receiver efficiency. For example, compare the two modulation formats in Fig. 1
Fig. 1 Illustration of two modulation formats: On/Off keyed (OOK) and 8-ary pulse position modulation (8-PPM). In PPM, the timing of the pulse encodes information. In the example shown, each pulse can occupy one of 8 time slots for a total of three bits of information.
. The top of Fig. 1 depicts simple ON/OFF keyed (OOK) modulation with a ½ mark ratio. Each symbol period codes a single bit of data. In contrast, pulse position modulation (PPM) transmits several bits of data with each symbol. The bottom of Fig. 1 illustrates (PPM), which is more photon-efficient but less bandwidth-efficient. In PPM, the optical energy is concentrated into a shorter pulse, requiring more bandwidth, and creating a large fraction of dead time. The timing of this short pulse then gives several bits of data: in Fig. 1, each pulse can occupy any of 8 timing slots to produce log2(8)=3 bits of data per symbol.

Under ideal circumstances, a photon counting PPM receiver could operate with arbitrarily few photons per data bit, down to even a tiny fraction of a photon on average for each data bit [2

2. J. Pierce, “Optical channels: practical limits with photon counting,” IEEE Trans. Commun. 26(12), 1819–1821 (1978). [CrossRef]

]. For example, a PPM signal with M slots per symbol and N photons per symbol could give up to N/log2(M) photons per bit; given unlimited bandwidth, we could make M arbitrarily large. Of course, bandwidth is limited, and the first limitation we reach is the electronic bandwidth required by the receiver to measure the arrival time of the PPM pulses. Assuming a data rate of b bits/s, OOK requires ~b Hz of bandwidth in the receiver. On the other hand, PPM requires ~bM/log2(M) Hz of bandwidth. For large M, this can strain electronic bandwidths, even at modest data rates (bandwidth is not the only non-ideality that could harm PPM performance; others include poor transmitter extinction, sources of background light, and high transmitted peak powers, which lead to nonlinear distortions in the transmitter).

One solution to the electronic bandwidth problem is to use a different orthogonal modulation format: frequency shift keying (FSK), shown in Fig. 2
Fig. 2 Illustration of frequency shift keying (FSK). A pulse is always present, and the carrier wavelength of each pulse codes information. In this 8-ary FSK case, the carrier frequency corresponding to the color blue codes the bits “110”, red codes “010”, and green codes “100”.
. In FSK, bits are coded by the carrier wavelength of the pulse rather than its position in time. So, by transmitting 1 carrier wavelength at a time, chosen from 8 carrier wavelengths, we can code log2(8)=3 bits of data per symbol. FSK’s power performance is the same as PPM; we are simply slicing data in frequency rather than time. But, the electronic bandwidth requirements are much lower than in PPM: b data bits/s requires only ~b/log2(M) Hz of electronic bandwidth, where M is the number of carrier wavelengths. For a general discussion of receiver sensitivity and modulation formats and their relationship to optical and electrical bandwidth, see [3

3. D. O. Caplan, “Laser communication transmitter and receiver design,” J. Opt. Fiber Comm. Res. 4(4-5), 225–362 (2007). [CrossRef]

].

Unfortunately, designs for optical FSK transmitters have not yet reached a mature commercial state. Many possible FSK transmitter designs, however, do exist in the literature. Perhaps the simplest FSK transmitter directly tunes a laser to different optical frequencies, but these transmitters generally do not have >GHz symbol rates and they rarely use FSK orders beyond about M=2 [4

4. R. S. Vodhanel, A. F. Elrefaie, M. Z. Iqbal, R. E. Wagner, J. L. Gimlett, and S. Tsuji, “Performance of directly modulated DFB lasers in 10-Gb/s ASK, FSK, and DPSK lightwave systems,” J. Lightwave Technol. 8(9), 1379–1386 (1990). [CrossRef]

11

11. J. Zhang, N. Chi, P. V. Holm-Nielsen, C. Peucheret, and P. Jeppesen, “An optical FSK transmitter based on an integrated DFB laser-EA modulator and its application in optical labeling,” IEEE Photon. Technol. Lett. 15(7), 984–986 (2003). [CrossRef]

]. In another class of FSK transmitters, a source generates a comb of optical frequencies, and then a tunable filter selects one at a time. MEMS based optical filters can have excellent extinction ratios, but cannot tune at rates far beyond 1 GHz [12

12. J.-W. Jeong, I. W. Jung, H. J. Jung, D. M. Baney, and O. Solgaard, “Multifunctional tunable optical filter using MEMS spatial light modulator,” J. Microelectromech. Syst. 19(3), 610–618 (2010). [CrossRef]

]. Micro-ring optical filters can be tuned by a GHz RF signal, but usually over a narrow range of optical wavelengths, and often with a low extinction ratio [13

13. P. Dong, R. Shafiiha, S. Liao, H. Liang, N.-N. Feng, D. Feng, G. Li, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, “Wavelength-tunable silicon microring modulator,” Opt. Express 18(11), 10941–10946 (2010). [CrossRef] [PubMed]

]. One integrated FSK transmitter uses fast-tuning lithium niobate modulators: a single optical frequency enters the integrated device, which then generates a comb of optical frequencies from the single input frequency. The same integrated device then selects one frequency in the comb for transmission. The comb of frequencies, however, is closely spaced at a few tens of GHz [14

14. M. Izutsu, S. Shikama, and T. Sueta, “Integrated optical SSB modulator/frequency shifter,” IEEE J. Quantum. Electron. 17(11), 2225–2227 (1981). [CrossRef]

, 15

15. T. Kawanishi, T. Sakamoto, T. Miyazaki, M. Izutsu, T. Fujita, S. Mori, K. Higuma, and J. Ichikawa, “High-speed optical DQPSK and FSK modulation using integrated Mach-Zehnder interferometers,” Opt. Express 14(10), 4469–4478 (2006). [CrossRef] [PubMed]

]. Our design is also a lithium niobate based device, but can operate on a widely spaced comb of optical frequencies. In our FSK transmitter, for M-ary FSK, the number of modulators grows only by log2(M), a favorable scaling for size, weight, and insertion loss. In experiment, we show that this design performs close to the theoretical optimum at data rates of 5 Gbit/s and 20 Gbit/s with 4-ary FSK (2 bits per symbol).

2. Capacity, bandwidth expansion, and power efficiency

3. FSK transmitter design

FSK can provide large gains in data rate and energy efficiency compared to OOK. The most obvious FSK transmitter using lithium niobate modulators requires M laser sources for the carrier frequencies, and M modulators to modulate those lasers. If instead a single tunable filter could select one of the M frequencies, the transmitter could cost less, be smaller, and use less power. Unfortunately, no commercial filter tunes at a ~10 GHz rate over 100s of GHz with ~10 GHz of bandwidth. Something similar, however, does exist. Figure 5
Fig. 5 Interferometric filter based on an imbalanced lithium niobate Mach-Zehnder interferometer.
shows a lithium niobate Mach-Zehnder tunable filter. By making one arm longer than the other, we can create an interferometric filter that passes every other carrier wavelength. Changing the voltage on an electrode in the other arm allows us to select which half to pass [18

18. E. L. Wooten, R. L. Stone, E. W. Miles, and E. M. Bradley, “Rapidly tunable narrowband wavelength filter using LiNbO3 unbalanced Mach-Zehnder interferometers,” J. Lightwave Technol. 14(11), 2530–2536 (1996). [CrossRef]

]. By cascading filters with different length imbalances in the two arms, each filter stage can block half of the remaining frequencies. So we need only log2(M) modulators rather than M. Figure 6
Fig. 6 FSK modulator using a cascade of interferometric filters.
shows a schematic of this kind of FSK transmitter.

We tested this design using electro-optic modulators with 20-GHz RF bandwidth, Vπ of 5 V at 20 GHz, and DC extinction ratio of 25 dB. Each stage of the transmitter used a separate electro-optic modulator, each custom produced by a commercial supplier. Our source was a bank of 4 DFB lasers from 192.2 to 192.5 THz, although lasers that produce many wavelengths at regular spacing exist in C-band [19

19. H. Takara, T. Ohara, K. Mori, K. Sato, E. Yamada, Y. Inoue, T. Shibata, M. Abe, T. Morioka, and K.-I. Sato, “More than 1000 channel optical frequency chain generation from single supercontinuum source with 12.5 GHz channel spacing,” Electron. Lett. 36(25), 2089–2090 (2000). [CrossRef]

]. The 1st stage filter had a single electrode, as shown in Fig. 5, and a filter periodicity of 100 GHz. The 2nd stage filter had a filter periodicity of 200 GHz and had two electrodes of two different lengths to tune it to one of 4 different operating voltages. A single electrode would have required the generation of a precise 4-level RF drive, which is difficult to do. With 2 electrodes, we were able to use two binary RF drives.

4. FSK transmitter performance

Unfortunately, the dynamic extinction ratio is difficult to measure, especially in an FSK signal. One method is to take a time-frequency spectrogram [3

3. D. O. Caplan, “Laser communication transmitter and receiver design,” J. Opt. Fiber Comm. Res. 4(4-5), 225–362 (2007). [CrossRef]

, 20

20. M. Kuznetsov and D. O. Caplan, “Time-frequency analysis of optical communication signals and the effects of second and third order dispersion,” in CLEO, Technical Digest (Optical Society of America, 2000).

]. Figure 8
Fig. 8 Spectrogram of the FSK signal produced by the transmitter. There are 4 carrier frequencies pulsing at 2.5 GHz.
shows a spectrogram of the 4-FSK transmitter operating at 2.5 Gsymbol/s. We collected the data by passing the transmitted signal through a tunable 3-GHz Fabry-Perot filter. We then measured the filtered signal on a 20-GHz photodiode. By sweeping the optical filter over the full range of FSK wavelengths, we obtained a spectrogram of the transmitted FSK signal. Figure 8 shows that the extinction ratio of each channel is always >20 dB, which corresponds to an ER-induced receiver penalty of less than ~0.5 dB.

We tested the bit error rate (BER) performance of the 4-FSK transmitter with the pre-amplified receiver described in [21

21. D. O. Caplan, J. J. Carney, and S. Constantine, “Parallel direct modulation laser transmitters for high-speed high-sensitivity laser communications,” in CLEO, Technical Digest (Optical Society of America, 2011).

]. An ideal pre-amplified receiver amplifies the FSK signal, separates and detects the 4 wavelengths, and chooses the wavelength with the largest signal, producing two bits of data for each received 4-ary symbol. Our receiver is a simplified sub-optimal version and is shown in Fig. 9
Fig. 9 Schematic of experiment testing performance of FSK transmitter and receiver pair. The experiment used 4-ary FSK at 2.5 and 10 Gsymbol/sec. The receiver detected light with optically pre-amplified photodiodes (PD). An arrayed waveguide grating (AWG) separated the light into separate wavelength channels. We measured the optical power at the input to the erbium-doped fiber amplifier.
. The pre-amplified FSK signal is separated into the 4 wavelengths using an arrayed waveguide grating (AWG) followed by a Fabry-Perot filter with peaks at the 4 wavelengths. These 4 wavelengths are then mixed in a mesh of couplers. At the output of this mesh, the summed power of wavelengths 1 and 2 is compared with the summed power of wavelengths 3 and 4, giving the most significant bit (MSB). Then the summed power of wavelengths 1 and 3 is compared with the summed power of wavelengths 2 and 4, giving the least significant bit (LSB).

Figure 10
Fig. 10 Bit error performance of the 4-ary FSK communications system as a function of the optical power collected at the receiver. The symbol rate is 2.5 GHz. Each symbol yields 2 bits of data, and each bit is shown on separate curves. The red curves show the performance of theoretically optimal pre-amplified (solid red) and photon counting receivers (dashed red).
shows the measured bit error probability of this system. Our transmitter produced an FSK signal with a symbol rate of 2.5 GHz, a data rate of 5 Gbit/s, and it used a 27-1 pseudorandom data pattern (all the components in our setup have good response at low frequencies, so longer patterns should perform just as well; our main concern is with high frequency response and rise time). Compared with the theoretically optimal pre-amplified system (solid red curve), our FSK system required between 3.3 and 4.1 dB more received power than a system with a theoretically optimal pre-amplified receiver. Reference [21

21. D. O. Caplan, J. J. Carney, and S. Constantine, “Parallel direct modulation laser transmitters for high-speed high-sensitivity laser communications,” in CLEO, Technical Digest (Optical Society of America, 2011).

] presents results for a similar experiment with 8-ary FSK at a 2.5 GHz symbol rate. That paper achieved a power penalty of 1.5 dB, better than our 3.3 to 4.1 dB penalty. We can account for most of this difference. First, the extinction ratio in [21

21. D. O. Caplan, J. J. Carney, and S. Constantine, “Parallel direct modulation laser transmitters for high-speed high-sensitivity laser communications,” in CLEO, Technical Digest (Optical Society of America, 2011).

] was >30 dB, better than our 20 dB. Poor extinction diverts gain in the transmitter away from the intended signal wavelength, and it also makes it harder for the receiver to choose the correct wavelength. Our 20 dB extinction leads to ~1 dB of additional penalty [22

22. D. O. Caplan, “Laser communication transmitter and receiver design,” J. Opt. Fiber Commun. Rep. 4(4-5), 225–362 (2007). [CrossRef]

]. Second, our filtering was different. In [21

21. D. O. Caplan, J. J. Carney, and S. Constantine, “Parallel direct modulation laser transmitters for high-speed high-sensitivity laser communications,” in CLEO, Technical Digest (Optical Society of America, 2011).

], the signal pulse and the filter profile were both approximately Gaussian, producing excellent matching. Our filter was a cascade of two Fabry-Perot filters, which gives a double-Lorentzian profile. This mismatch leads to at least 0.3 dB of additional penalty [22

22. D. O. Caplan, “Laser communication transmitter and receiver design,” J. Opt. Fiber Commun. Rep. 4(4-5), 225–362 (2007). [CrossRef]

]. That still leaves us with approximately 0.5 to 1.3 dB of extra penalty. Much of this penalty is probably due to the variability in the peak pulse intensities in Fig. 8, a problem that is absent from the parallel setup of [21

21. D. O. Caplan, J. J. Carney, and S. Constantine, “Parallel direct modulation laser transmitters for high-speed high-sensitivity laser communications,” in CLEO, Technical Digest (Optical Society of America, 2011).

]. Some additional penalty may be due to differences in EDFA noise figure and other variability in components.

We also tested the error probabilities at a symbol rate of 10 GHz, giving a data rate of 20 Gbit/s, shown in Fig. 11
Fig. 11 Bit error performance of the 4-ary FSK system at a symbol rate of 10 GHz.
. This time, our FSK system required from 4.4 to 7.5 dB more received power. The additional penalty was partly caused by a reduced extinction ratio in each channel at the higher rate, which was typically ~20 dB, but sometimes closer to 15 dB.

5. Photon counting receivers

As seen in Figs. 10 and 11, photon counting receivers can in theory perform better than pre-amplified receivers, and this improvement is very great at large M. To explore this potential, we tested the FSK signal on a receiver using superconducting nanowire photon counters instead of optically pre-amplified photodiodes [23

23. G. N. Gol’tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, “Picosecond superconducting single-photon optical detector,” Appl. Phys. Lett. 79(6), 705–707 (2001). [CrossRef]

, 24

24. E. A. Dauler, B. S. Robinson, A. J. Kerman, J. K. W. Yang, E. K. M. Rosfjord, V. Anant, B. Voronov, G. Gol’tsman, and K. K. Berggren, “Multi-element superconducting nanowire single-photon detector,” IEEE Trans. Appl. Supercond. 17(2), 279–284 (2007). [CrossRef]

]. We had to test at a lower rate – 100 MHz – because of these detectors’ long reset times.

Our goal in this experiment was not simply to measure the error rate of an uncoded FSK link. After all, we chose photon counters to construct the most power efficient system, and the most efficient system would also use forward error correcting (FEC) codes. We did not have the resources and equipment needed to construct a receiver capable of decoding an FEC in real time. Nonetheless, we can infer ideal coded performance, measured by the number of photons received per bit received, from easily measured statistics: 1) the probabilities of each wavelength being transmitted (in our case, all wavelengths were equally likely); and 2) the transition probabilities that a pulse transmitted on wavelength k was received as a pulse on wavelength i, or was erased altogether. Table 1

Table 1. Probabilities that the receiver detected the transmitted wavelength, or a different wavelength.

table-icon
View This Table
shows the probabilities that we measured. We took real time oscilloscope traces of the FSK transmitter’s output. We know the transmitted data pattern, so, from these scope traces, we can measure how often our received signal matches the transmitted one.

We can easily express and understand the mutual information in terms of the information entropy, which measures the number of bits of uncertainty carried by a random variable, X: H(X)=ipilog2(pi), where X is a random variable of all possible transmitted symbols, and pi is the probability that X takes on value xi. In the simplest case, each symbol can take on one of two equally possible wavelengths, so pi equals ½ for i=1,2, and H(X)=1 bit per symbol. This matches our intuition: a signal that is in one of two equally possible states should carry 1 bit of uncertainty (i.e., information). For an 8-ary FSK system, we expect that the transmitted signal carries 3 bits of uncertainty, and indeed pi=1/8 for i=1to8, giving H(X)=3 bits.

Of course, so far we only know the information in our transmitted signal, X, but we want to know what information we can obtain from our received signal, Y. Ymay be corrupted by noise: if we transmit xi, we want to receive yi, but may receive yk instead. One useful quantity to consider is H(X|Y=yk)=ipi|klog2(pi|k), where pi|k is a conditional probability that X=xi given that we receive Y=yk. Ideally, pi|k=1 when i=k and is 0 otherwise, which means we receive exactly the information that was sent. If so, then H(X|Y=yk)=0. Thus H, as a measure of uncertainty, is telling us that knowing the received signal, Y=yk, means that we know the transmitted signal, X, with certainty (i.e., with 0 uncertainty), which is the goal of any communications system. In reality, there is some chance that the signal is received incorrectly, or is erased all together, so that pi|k is not as ideal as above. Now, we’re interested in all possible received signals, Y, so we take the expected value of H(X|Y=yk), averaged over all k, to obtain a new quantity, H(X|Y)=E[H(X|Y=yk)]. Similar to H(X|Y=yk) above, H(X|Y) measures, in bits per symbol, the remaining uncertainty in X after measuring Y. A communications receiver tries to remove all uncertainty in X, so that it can say confidently from a measurement of Y what the value of X was. Thus we would like H(X|Y) to be as close to 0 as possible. From this, we arrive at the definition for mutual information: I(X;Y)H(X)H(X|Y). If H(X|Y)=0, we know X with certainty from our measurement of Y. Thus we get I(X;Y)=H(X), the maximum possible number of bits per symbol. When H(X|Y)>0, some of the transmitted bits will have to be error correcting bits, reducing the number of information bits, I(X;Y), we get per symbol. From measurements of the probabilities, like pi|k, we can calculateI(X;Y).

As mentioned before, the detectors we used, though fast for photon counters, limited us to 100 Msymbol/s. Higher symbol rates are possible by using arrays of photon counters to counteract the reset time [26

26. S. Verghese, J. P. Donnelly, E. K. Duerr, K. A. McIntosh, D. C. Chapman, C. J. Vineis, G. M. Smith, J. E. Funk, K. E. Jensen, P. I. Hopman, D. C. Shaver, B. F. Aull, J. C. Aversa, J. P. Frechette, J. B. Glettler, Z. L. Liau, J. M. Mahan, L. J. Mahoney, and K. M. Molvar, “Arrays of InP-based avalanche photodiodes for photon counting,” IEEE J. Sel. Top. Quantum Electron. 13, 870–886 (2007). [CrossRef]

]. Such arrays could detect FSK signals at Gsymbol/s rates. Moreover, we could add carrier wavelengths to expand the data rate further – each doubling of the number of carriers adds a bit of data per symbol. In contrast, the same process in PPM requires a doubling of the number of time slots, and thus a doubling of electronic rates.

5. Conclusions

Acknowledgment

References and links

1.

A. Biswas, D. Boroson, and B. Edwards, “Mars laser communication demonstration: what it would have been,” Proc. SPIE6105, 610502 (2006).

2.

J. Pierce, “Optical channels: practical limits with photon counting,” IEEE Trans. Commun. 26(12), 1819–1821 (1978). [CrossRef]

3.

D. O. Caplan, “Laser communication transmitter and receiver design,” J. Opt. Fiber Comm. Res. 4(4-5), 225–362 (2007). [CrossRef]

4.

R. S. Vodhanel, A. F. Elrefaie, M. Z. Iqbal, R. E. Wagner, J. L. Gimlett, and S. Tsuji, “Performance of directly modulated DFB lasers in 10-Gb/s ASK, FSK, and DPSK lightwave systems,” J. Lightwave Technol. 8(9), 1379–1386 (1990). [CrossRef]

5.

S. B. Alexander, R. Barry, D. M. Castagnozzi, V. W. S. Chan, D. M. Hodsdon, L. L. Jeromin, J. E. Kaufmann, D. M. Materna, R. J. Parr, M. L. Stevens, and D. W. White, “4-ary FSK coherent optical communication system,” Electron. Lett. 26(17), 1346–1348 (1990). [CrossRef]

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R. Noe, H. Rodler, A. Ebberg, G. Gaukel, and F. Auracher, “Optical FSK transmission with pattern independent 119 photoelectrons/bit receiver sensitivity with endless polarization control,” Electron. Lett. 25(12), 757–758 (1989). [CrossRef]

7.

A. H. Gnauck, K. C. Reichmann, J. M. Kahn, S. K. Korotky, J. J. Veselka, and T. L. Koch, “4-Gb/s heterodyne transmission experiments using ASK, FSK and DPSK modulation,” IEEE Photon. Technol. Lett. 2(12), 908–910 (1990). [CrossRef]

8.

R. S. Vodhanel, J. L. Gimlett, N. K. Cheung, and S. Tsuji, “FSK heterodyne transmission experiments at 560 Mbit/s and 1 Gbit/s,” J. Lightwave Technol. 5(4), 461–468 (1987). [CrossRef]

9.

A. R. Chraplyvy, R. W. Tkach, A. H. Gnauck, and R. M. Derosier, “8Gbit/s FSK modulation of DFB lasers with optical demodulation,” Electron. Lett. 25(5), 319–321 (1989). [CrossRef]

10.

J. Zhang, N. Chi, P. V. Holm-Nielsen, C. Peucheret, and P. Jeppesen, “An optical FSK transmitter based on an integrated DFB laser-EA modulator and its application in optical labeling,” IEEE Photon. Technol. Lett. 15(7), 984–986 (2003). [CrossRef]

11.

J. Zhang, N. Chi, P. V. Holm-Nielsen, C. Peucheret, and P. Jeppesen, “An optical FSK transmitter based on an integrated DFB laser-EA modulator and its application in optical labeling,” IEEE Photon. Technol. Lett. 15(7), 984–986 (2003). [CrossRef]

12.

J.-W. Jeong, I. W. Jung, H. J. Jung, D. M. Baney, and O. Solgaard, “Multifunctional tunable optical filter using MEMS spatial light modulator,” J. Microelectromech. Syst. 19(3), 610–618 (2010). [CrossRef]

13.

P. Dong, R. Shafiiha, S. Liao, H. Liang, N.-N. Feng, D. Feng, G. Li, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, “Wavelength-tunable silicon microring modulator,” Opt. Express 18(11), 10941–10946 (2010). [CrossRef] [PubMed]

14.

M. Izutsu, S. Shikama, and T. Sueta, “Integrated optical SSB modulator/frequency shifter,” IEEE J. Quantum. Electron. 17(11), 2225–2227 (1981). [CrossRef]

15.

T. Kawanishi, T. Sakamoto, T. Miyazaki, M. Izutsu, T. Fujita, S. Mori, K. Higuma, and J. Ichikawa, “High-speed optical DQPSK and FSK modulation using integrated Mach-Zehnder interferometers,” Opt. Express 14(10), 4469–4478 (2006). [CrossRef] [PubMed]

16.

B. Robinson and D. Boroson, “Achievable capacity using photon-counting array-based receivers with on-off-keyed and frequency-shift-keyed modulation formats,” Proc. SPIE 8246, 824604 (2012). [CrossRef]

17.

D. Boroson, “A survey of technology-driven capacity limits for free-space laser communications,” Proc. SPIE 6709, 670918, 670918-19 (2007). [CrossRef]

18.

E. L. Wooten, R. L. Stone, E. W. Miles, and E. M. Bradley, “Rapidly tunable narrowband wavelength filter using LiNbO3 unbalanced Mach-Zehnder interferometers,” J. Lightwave Technol. 14(11), 2530–2536 (1996). [CrossRef]

19.

H. Takara, T. Ohara, K. Mori, K. Sato, E. Yamada, Y. Inoue, T. Shibata, M. Abe, T. Morioka, and K.-I. Sato, “More than 1000 channel optical frequency chain generation from single supercontinuum source with 12.5 GHz channel spacing,” Electron. Lett. 36(25), 2089–2090 (2000). [CrossRef]

20.

M. Kuznetsov and D. O. Caplan, “Time-frequency analysis of optical communication signals and the effects of second and third order dispersion,” in CLEO, Technical Digest (Optical Society of America, 2000).

21.

D. O. Caplan, J. J. Carney, and S. Constantine, “Parallel direct modulation laser transmitters for high-speed high-sensitivity laser communications,” in CLEO, Technical Digest (Optical Society of America, 2011).

22.

D. O. Caplan, “Laser communication transmitter and receiver design,” J. Opt. Fiber Commun. Rep. 4(4-5), 225–362 (2007). [CrossRef]

23.

G. N. Gol’tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, “Picosecond superconducting single-photon optical detector,” Appl. Phys. Lett. 79(6), 705–707 (2001). [CrossRef]

24.

E. A. Dauler, B. S. Robinson, A. J. Kerman, J. K. W. Yang, E. K. M. Rosfjord, V. Anant, B. Voronov, G. Gol’tsman, and K. K. Berggren, “Multi-element superconducting nanowire single-photon detector,” IEEE Trans. Appl. Supercond. 17(2), 279–284 (2007). [CrossRef]

25.

T. M. Cover and J. A. Thomas, Elements of Information Theory (Wiley-Interscience, 2005).

26.

S. Verghese, J. P. Donnelly, E. K. Duerr, K. A. McIntosh, D. C. Chapman, C. J. Vineis, G. M. Smith, J. E. Funk, K. E. Jensen, P. I. Hopman, D. C. Shaver, B. F. Aull, J. C. Aversa, J. P. Frechette, J. B. Glettler, Z. L. Liau, J. M. Mahan, L. J. Mahoney, and K. M. Molvar, “Arrays of InP-based avalanche photodiodes for photon counting,” IEEE J. Sel. Top. Quantum Electron. 13, 870–886 (2007). [CrossRef]

OCIS Codes
(060.4510) Fiber optics and optical communications : Optical communications
(060.2605) Fiber optics and optical communications : Free-space optical communication
(250.4110) Optoelectronics : Modulators

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: October 15, 2012
Revised Manuscript: December 21, 2012
Manuscript Accepted: January 7, 2013
Published: February 4, 2013

Citation
Shelby Jay Savage, Bryan S. Robinson, David O. Caplan, John J. Carney, Don M. Boroson, Farhad Hakimi, Scott A. Hamilton, John D. Moores, and Marius A. Albota, "Scalable modulator for frequency shift keying in free space optical communications," Opt. Express 21, 3342-3353 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-3-3342


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