Efficient communication at telecom wavelengths using wavelength conversion and silicon photon-counting detectors

Transcription

1 Efficient communication at telecom wavelengths using wavelength conversion and silicon photon-counting detectors M. E. Grein* a, L. E. Elgin a, B. S. Robinson a a a, David O. Caplan, Mark L. Stevens, S. A. Hamilton a, D. M. Boroson a, C. Langrock b, M. M. Fejer b a Massachusetts Institute of Technology, Lincoln Laboratory, 244 Wood St., Lexington, MA 02420, USA; b Stanford University, Edward L. Ginzton Laboratory, Stanford, CA , USA ABSTRACT Silicon Geiger-mode avalanche photodiodes (Si GM-APDs) have desirable properties for an optical photon-counting receiver, including high single-photon detection efficiency, low reset time, and low timing jitter; however, they do not detect near-ir photons. In this work, we demonstrated a sensitive photon-counting receiver in the near-ir by combining a wavelength converter consisting of a periodically-poled lithium niobate (PPLN) waveguide and a commercial Si GM-APD detector. We measured a receiver sensitivity from 1.4 to 3.5 incident photons/bit from 5.5 Mb/s to 22 Mb/s for a single detector, and achieved a sensitivity of 4 photons/bit at 78 Mb/s using an emulated array of 25 detectors. Keywords: photon counting, periodically-poled lithium niobate, optical communications, silicon Geiger-mode avalanche photodiodes 1. INTRODUCTION Free-space communication links for deep-space and near-earth present a number of technical challenges. Among them are excessive diffractive losses due to the large distances between transmitter and receiver and the limited resources highly constrained by the satellite/space environment, including transmitter and receiver aperture size and weight and limited transmitter power. Employing a sensitive optical receiver reduces the required transmitter power. It has been shown that an optical communications link employing a pulse-position modulation (PPM) format and a photon-counting receiver has the potential to achieve ultra-high (< 1 photon/bit) sensitivity[1]. With PPM, a symbol, representing log 2 (M) bits of information, is transmitted by sending a single pulse in one of M transmission slots. To date, however, the availability of high-performance photon counting receivers in the NIR wavelength range has been limited. Superconducting NbN nanowires have been shown to achieve 1.25 Gb/s at 1550 nm but require cryogenic cooling[2]. For deep-space applications requiring high-sensitivity at data rates of tens or hundreds of Mb/s, there are alternative approaches using photomultiplier tubes[3] and InGaAs Geiger-mode avalanche photodiodes (GM-APDs)[4], where the latter achieved better than 1.4 incident photon/bit sensitivity at 14 Mb/s. In both cases, the performance was limited by the detectors. In comparison, commercially-available Si GM-APDs have very favorable properties that can support high data rates while achieving high sensitivity but only in the nm band. By cascading a wavelength converting periodically-poled lithium niobate (PPLN) waveguide in front of a Si GM-APD, we demonstrated a photon-counting receiver detecting with better than 1.4 incident photons/bit at 5.5 Mb/s and better than 3.5 photons/bit at 20 Mb/s at 1550 nm[5]. The data rate was primarily limited by blocking due to the ~50 ns reset time and the timing jitter of the Si GM- APD. The blocking penalty was overcome using a 5x5 array of Si GM-APDs (in this case, the 25-element array was emulated using a single Si GM-APD device) to achieve error-free performance at 78 Mb/s. This represents an increase of a factor of four over previously reported results using a wavelength converter and a Si GM-APD[5] and has the potential to improve by another order of magnitude. In section two, the wavelength converter is described, and the optical communications testbed is discussed in section three with a summary following thereafter. *megrein@ll.mit.edu; phone ; fax This work is sponsored by National Aeronautics and Space Administration under Air Force Contract #FA C Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by the United States Government.

2 2. WAVELENGTH CONVERSION USING PPLN The PPLN-based waveguide upconverter has been described in previous publications[5-7] and shown in Figure 1. Incoming signal photons at 1556 nm are combined with continuous-wave 1319 nm pump photons through a fiber wavelength-division-multiplexer and copropagate in a 48-mm long temperature-stabilized PPLN waveguide. The grating poling period written in the waveguide promotes the conversion of 1556 nm photons to 713 nm photons using a Signal 1550 nm Periodically-Poled LiNbO 3 Waveguide Coupler Quartz Prism Iris Nd:YAG, 1319 nm Filter Bandpass Filter Upconverted Signal, 712 nm Fig 1. Wavelength upconverter based on periodically-poled lithium-niobate (PPLN) nm pump. Filtering of the pump laser s second-harmonic was achieved spatially (with a SF10 Brewster prism and iris) and spectrally (with a 5 nm bandpass filter). The filtered light was coupled into a multimode fiber-coupled Si GM- APD (~65% detection efficiency, dark count rate 450 Hz). The external upconversion efficiency (number of converted photons between the waveguide input/output), shown in Fig. 2, reached a maximum of 60.8% with 108 mw of pump power. In future waveguide designs, the external upconversion efficiency has the potential to achieve >90%[6-7]. The dark counts also increased with the pump power, suggestive of a pump-induced nonlinear process but did not prove to be a limiting factor in our experiments. External Conversion Efficiency, % Dark Counts, khz Pump Power, mw Fig. 2 External upconversion efficiency of the PPLN waveguide (circles) with a fit to the theory, and pump-induced dark count rate (squares) with an empirical fit to the square of the pump power.

3 3. OPTICAL COMMUNICATION EXPERIMENTS The optical communication testbed for testing the photon-counting receiver is shown in Fig. 3. A DFB cw laser near 1556 nm is modulated by a pair of lithium-niobate Mach-Zehnder intensity modulators achieving an on/off extinction ratio >60 db[8]. The data is encoded with a ½-rate serially-concatenated PPM (SCPPM) forward error correction code with M slots per symbol period and slot widths corresponding to 1/f slot. For M = 128, 5 uncoded bits are transmitted per symbol (3.5 coded bits per symbol). The data is then attenuated before reaching the photon-counting receiver. The Transmitter Slot Clock Receiver DFB Laser PPM Formatter Modulator Data Power Amp Attn PPLN-based Photon Counter Clock Recovery and Synch Demodulator Data Fig 3. Optical communications testbed employing pulse-position modulation (PPM) in the transmitter and a photoncounting receiver. The photon counter is based on a periodically-poled lithium niobate (PPLN) wavelength converter and a silicon Geiger-mode avalanche photodiode. photon counter consists of the PPLN waveguide wavelength converter of Fig. 1 and a Si GM-APD (here, a commercial Perkin Elmer SPCM AQR-12-FC). The detected pulses are then demodulated and the bit-error-rate is calculated. For the experiments shown in this paper, the slot clock is transmitted to the receiver for synchronization of the transmitter and receiver. The bit-error rate (BER) for the case of a 200 MHz slot clock (5 ns slots) and M = 128 is shown in Fig. 4 as a function of power incident to the receiver in units of photons/bit. The uncoded data follows that of a Poisson channel Bit-Error-Rate Uncoded Data Coded Data Capacity Turbo code 1.7 db performance Implementation loss 6.9 db Incident photons/bit, db Fig 4. Bit-error rate performance of the PPLN-based photon-counting receiver using a single silicon Geiger-mode avalanche photodiode. The slot clock rate, 200 MHz; the constellation M, 128 (yielding 3.5 coded bits per symbol with a 1/2 rate SCPPM FEC). With the FEC code, the received power corresponding to error-free performance is 1.4 photons/bit (alternatively, 1.6 db photons/bit) at a data rate of 5.5 Mb/s. This represents one of the most sensitive reported demonstrations of a photoncounting receiver in the NIR wavelength range at Mb/s-class data rates. The performance of the receiver is compared to the theoretical capacity of a background-free PPM erasure channel with Poisson statistics, shown by the solid line in Fig.

4 4. The SCPPM FEC code performance was 1.7 db from theory. Unlike an optically-preamplified receiver, the receiver losses directly impact the sensitivity. The 6.9 db implementation loss includes linear coupling loss into and out of the PPLN waveguide, Fresnel losses at the waveguide interfaces, filtering losses, coupling losses onto the Si GM-APD, and ~65% detection efficiency of the Si GM-APD. With some modest improvements to the receiver design, including ARcoating of the PPLN waveguide and better mode matching between the waveguide modes and optical fiber modes, and employing a low-loss optical filter, the sensitivity can be increased by an additional 3 db or more. The data rate was varied by changing both the number of slots per symbol, M, and the slot width. The data rate, f data, is given by f 1 = f 2 log2 M M data slot (1) where f slot is the slot clock (with a slot width of 1/f slot ) and M the number of slots per symbol period. One can increase the data rate by reducing the number of slots per symbol (smaller M) and increasing the slot rate, but reducing M reduces the sensitivity which is proportional to log 2 M. Additionally, since M must have a minimum value of two, going to higher data rates requires increasing the slot-clock frequency (reducing the slot-clock width). Figure 5 shows the results of the receiver performance over a range of data rates. The measure of sensitivity corresponds to the power incident to the receiver (in photons/bit) where the BER has reached the error-free cliff. As seen by the data (filled squares) in Fig. 5, the receiver sensitivity degraded by 2.1 db when the data rate was varied from 5.5 Mb/s to 22 Mb/s. Potential factors contributing to the performance penalty are blocking and timing jitter of the Si GM-APD. After the Si GM-APD has 10 Sensitivity (photons/bit) Data Rate (Mb/s) Fig. 5. Sensitivity (measured in photons/bit incident to the receiver) at a given data rate. Filled squares, PPLN-based receiver with a single Si GM-APD; filled circles, emulated array of PPLN-based receiver with 25 Si GM-APDs; unfilled squares and unfilled circles, theoretical capacity for an ideal, noiseless photon-counting receiver. fired following the detection of a photon, the detector requires a minimum time to reset before detecting another photon. Any photons arriving during the reset time are effectively blocked, resulting in a receiver penalty. This effect can be mitigated by using an array of individually-addressed photon-counting detectors[4]. While one detector fires and is resetting, the others in the array are active and can still receive a photon. The timing jitter of the Si GM-APD is due to the statistical randomness governing the absorption of the photon and subsequent avalanching events that produce a current pulse[9-11]. The effects of timing jitter increase as the slot clock width becomes comparable to the timing jitter[12] and limits the maximum-achievable data rate. The filled circles of Fig. 5 show the performance of a 25- element array of Si GM-APDs (emulated using a single Si GM-APD, attenuating the upconverted light by a factor of 25, and repeating the code words 25 times). Going to the array relieved much of the blocking penalty associated with using only a single detector. The penalty due to timing-jitter was the dominant cause for the sensitivity decrease observed in Fig. 5 for both the single Si GM-APD and the array of 25 Si GM-APDs, and the maximum data rate achieved for the emulated array was 78 Mb/s with a sensitivity of 4.0 photons/bit.

5 4. SUMMARY AND CONCLUSIONS A photon-counting receiver has been developed using a PPLN waveguide upconverter and a commercial Si GM-APD. With a single Si GM-APD and using a SCPPM FEC coding scheme, a receiver sensitivity of 1.4 incident photons/bit at 5.5 Mb/s and 3.5 photons/bit at 22 Mb/s have been achieved. Using an emulated array of 25 Si GM-APDs, the data rate was increased to 78 Mb/s with a sensitivity of 4.0 photons/bit. The limitation to the data rate is currently limited by the timing jitter of the Si GM-APD. The Perkin Elmer type of Si GM-APD have a thick Si absorption region, lending to the high (~ 65%) single-photon detection efficiency but leading to a relatively large timing jitter. By thinning the absorber region using a planarized design, the timing jitter can be much smaller but at the expense of detection efficiency. REFERENCES 1. J. R. Pierce, IEEE Trans. Communications COM-26, (1978). 2. E. A. Dauler, B.S. Robinson, A.J. Kerman, V. Anant, R. J. Barron, K. K. Berggren, D. O. Caplan, J. J. Carney, S. A. Hamilton, K. M. Rosfjord, M. L. Stevens, and J. K. W. Yang, Advanced Photon Counting Techniques, edited by Wolfgang Becker, Proc. of SPIE, Vol. 6372, (2006). 3. A. Biswas and W. Farr, TDA Progress Report (2003). 4. P. I. Hopman, P. W. Boettcher, L. M. Candell, J. B. Glettler, R. Shoup, and G. Zobgi, Proc. SPIE Vol. 6304, paper 63040H, Free-Space Laser Communications VI; Arun K. Majumdar, Christopher C. Davis; Eds. (2006). 5. M. E. Grein, L. E. Elgin, B. S. Robinson, S. A. Hamilton, D. M. Boroson, C. Langrock, and M. M. Fejer, in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies 2006 Technical Digest (Optical Society of America, Washington, DC 2006), paper CFH4. 6. R. V. Roussev, C. Langrock, J. R. Kurz, and M. M. Fejer, Opt. Lett. 29, (2004). 7. C. Langrock, E. Diamant et al, Opt. Lett. 30, (2005). 8. B. S. Robinson, D. O. Caplan, M. L. Stevens, R. J. Barron, E. A. Dauler, and S. A. Hamilton, in 2005 Digest of the LEOS Summer Topical Meetings (IEEE, 2005), paper TuA P. P. Webb and R. J. McIntyre, RCA Engineer 27 (1982). 10. G. Ripamonti and S. Cova, Solid State Electron 28, (1985). 11. A. Lacaita M. Mastrapasqua, M. Ghioni, and S. Vanoli, APL 57 (1990). 12. A. Kachelmeyer, MIT Lincoln Laboratory, in press (2007).