## Polarization insensitive 25-Gbaud direct D(Q)PSK receiver based on polymer planar lightwave hybrid integration platform |

Optics Express, Vol. 19, Issue 13, pp. 12197-12207 (2011)

http://dx.doi.org/10.1364/OE.19.012197

Acrobat PDF (1469 KB)

### Abstract

We report a direct DPSK receiver based on polymer planar lightwave circuit technology, which incorporates a 2x25 GHz photodiode (PD) array hybridly integrated via 45° mirrors. In this direct DPSK receiver, a half-wave plate and heating electrodes are implemented to eliminate the polarization-dependent frequency-shift (PDFS) of the delay-line interferometer (DLI). By applying a proper heating current, a residual PDFS of practically zero at 1550 nm and within ±125 MHz was achieved over the full C-band. Integrated with the PD array, the peak responsivity is ~0.14 A/W for orthogonal polarizations over the C-band. To characterize this direct receiver, we introduce an adapted common-mode rejection ratio (CMRR), which takes into account the unequal responsivities of the PDs, the uneven split of the input power by the DLI, the phase error and the extinction ratio of the DLI. The measured CMRR under DC condition is below −20 dB over the C-band.

© 2011 OSA

## 1. Introduction

1. A. Gnauck and P. Winzer, “Optical phase-shift-keyed transmission,” J. Lightwave Technol. **23**(1), 115–130 (2005). [CrossRef]

1. A. Gnauck and P. Winzer, “Optical phase-shift-keyed transmission,” J. Lightwave Technol. **23**(1), 115–130 (2005). [CrossRef]

2. H. Kim and P. Winzer, “Robustness to laser frequency offset in direct detection DPSK and DQPSK systems,” J. Lightwave Technol. **21**(9), 1887–1891 (2003). [CrossRef]

2. H. Kim and P. Winzer, “Robustness to laser frequency offset in direct detection DPSK and DQPSK systems,” J. Lightwave Technol. **21**(9), 1887–1891 (2003). [CrossRef]

*n*

_{TE}−

*n*

_{TM}) of ~±1×10

^{−6}/±2×10

^{−6}, where

*n*

_{TE}and

*n*

_{TM}are the refractive indices for the TE (transverse electric) and TM (transverse magnetic) polarization states. Note that the PDFS is defined as frequency shift between TE and TM modes, Fig. 1(b). This definition takes into account that the receiver reported in this work has been designed for use in dual-polarization transmission systems, where the main concern is about how much the transmission functions for the TE and TM states differ.

^{−5}has been achieved [3

3. C. R. Doerr, D. M. Gill, A. H. Gnauck, L. I. Buhl, P. J. Winzer, M. A. Cappuzzo, A. Wong-Foy, E. Y. Chen, and L. T. Gomez, “Monolithic demonstrator for 40 Gb/s DQPSK using a star coupler,” J. Lightwave Technol. **24**(1), 171–174 (2006). [CrossRef]

8. J. Gamet and G. Pandraud, “C- and L-Band planar delay interferometer for DPSK decoders,” IEEE Photon. Technol. Lett. **17**(6), 1217–1219 (2005). [CrossRef]

9. H. H. Yaffe, C. H. Henry, R. F. Kazarinov, and M. A. Milbrodt, “Polarization-independent silica-on-silicon Mach-Zehnder Interferometers,” J. Lightwave Technol. **12**(1), 64–67 (1994). [CrossRef]

12. L. Eldada and L. W. Shachlette, “Advances in polymer integrated optics,” IEEE J. Sel. Top. Quantum Electron. **6**(1), 54–68 (2000). [CrossRef]

13. H. Ma, A. K.-Y. Jen, and L. R. Dalton, “Polymer-based optical waveguides: materials, processing, and devices,” Adv. Mater. (Deerfield Beach Fla.) **14**(19), 1339–1365 (2002). [CrossRef]

^{−4}range [12

12. L. Eldada and L. W. Shachlette, “Advances in polymer integrated optics,” IEEE J. Sel. Top. Quantum Electron. **6**(1), 54–68 (2000). [CrossRef]

13. H. Ma, A. K.-Y. Jen, and L. R. Dalton, “Polymer-based optical waveguides: materials, processing, and devices,” Adv. Mater. (Deerfield Beach Fla.) **14**(19), 1339–1365 (2002). [CrossRef]

15. Technical documentations, ZPU12-RI & ZPU13-RI (UV curable polymers), ChemOptics co., Korea. http://inct.raonnet.com/admin_e/pageMake_proto.php?a_name=VGVjaG5vbG9neSBEb2N1bWVudHM=&aa_code=1214.

## 2. Compensation of polarization dependent frequency shift (PDFS)

### 2.1 Configuration and operation principle

*L*+ Δ

*L*and

*L*; the lengths of the strip heaters are δ

*L*

_{1}and δ

*L*

_{2}for the upper and lower path, while δ

*L*

_{2}= δ

*L*

_{1}⋅

*L*/(

*L*+ Δ

*L*).

*n*

_{g,ν}is the effective group refractive index for different states of polarization, and Δ

*L*is the length difference between the DLI arms. Due to the birefringence, the power transfer function has a dependence on the state of polarization, i.e. PDFS.

*B*

_{WG}is in the order of 10

^{−3}[15

15. Technical documentations, ZPU12-RI & ZPU13-RI (UV curable polymers), ChemOptics co., Korea. http://inct.raonnet.com/admin_e/pageMake_proto.php?a_name=VGVjaG5vbG9neSBEb2N1bWVudHM=&aa_code=1214.

13. H. Ma, A. K.-Y. Jen, and L. R. Dalton, “Polymer-based optical waveguides: materials, processing, and devices,” Adv. Mater. (Deerfield Beach Fla.) **14**(19), 1339–1365 (2002). [CrossRef]

*S*=

*B*

_{WG}/Δ

*T*

_{Proc}, where Δ

*T*

_{Proc}=

*T*

_{f}–

*T*

_{0}, with

*T*

_{f}denoting the final temperature in the process and

*T*

_{0}the room temperature. The value of

*S*is always negative and in the order of 10

^{−5}/

*K*for polymer waveguides because Δ

*T*

_{Proc}is in the order of 100 – 200

*K*.

^{−5}can be obtained by using a half-wave plate [3

3. C. R. Doerr, D. M. Gill, A. H. Gnauck, L. I. Buhl, P. J. Winzer, M. A. Cappuzzo, A. Wong-Foy, E. Y. Chen, and L. T. Gomez, “Monolithic demonstrator for 40 Gb/s DQPSK using a star coupler,” J. Lightwave Technol. **24**(1), 171–174 (2006). [CrossRef]

^{−6}as follows: If the waveguide temperature is increased from room temperature

*T*

_{0}to

*T*

_{1}by using the left heaters or to

*T*

_{2}by using the heaters on the right, the waveguide birefringence decreases by a value δ

*B*

_{1}=

*S*(

*T*

_{1}−

*T*

_{0}) or δ

*B*

_{2}=

*S*(

*T*

_{2}−

*T*

_{0}), respectively. For the waveguides under the trimming heaters with a length of δ

*L*

_{1}on the top-left and top-right DLI sections, Fig. 2(a), the corresponding phase changes are Δ

*ϕ*

_{1}and Δ

*ϕ*

_{2}. Their values and their difference Δ

*ϕ*

_{2}−Δ

*ϕ*

_{1}are calculated as:

*B*

_{res}of 10

^{−5}, can be considered as a residual phase difference Δ

*ϕ*

_{res}. For a path-length of the longer DLI arm

*L*+ Δ

*L*, we get

*S*=

*B*

_{WG}/Δ

*T*

_{Proc}, the required temperature difference between the right and left heating element is then

*B*

_{res},

*T*

_{2}−

*T*

_{1}can be positive or negative, by heating the left and right electrodes separately. As an estimation,

*B*

_{res}/

*B*

_{WG}is in the order of 10

^{−2}~10

^{−3}, Δ

*T*

_{proc}is in the range of 100 – 200

*K*, and the length factor (

*L*+ Δ

*L*)/δ

*L*

_{1}is in the order of 10, it can be seen that a temperature difference of only a few degrees is sufficient to fully compensate the residual birefringence of the DLI.

*T*

_{DLI}, the required temperature difference for compensating PDFS is Δ

*T*=

*T*

_{2}−

*T*

_{1}, then the left and right heating electrodes should be operated at

*T*

_{DLI}– Δ

*T*/2 and

*T*

_{DLI}+ Δ

*T*/2, respectively. This means that no extra structure will be needed, thus leading to a compact, low electric power consumption, and low cost device.

*L*

_{2}= δ

*L*

_{1}⋅

*L*/(

*L*+ Δ

*L*).

### 2.2. Results of PDFS compensating method

15. Technical documentations, ZPU12-RI & ZPU13-RI (UV curable polymers), ChemOptics co., Korea. http://inct.raonnet.com/admin_e/pageMake_proto.php?a_name=VGVjaG5vbG9neSBEb2N1bWVudHM=&aa_code=1214.

*L*

_{1}of 3.06 mm and a δ

*L*

_{2}of 2.23 mm.

4. C. R. Doerr, M. A. Cappuzzo, E. Y. Chen, A. Wong-Foy, L. T. Gomez, S. S. Patel, S. Chandrasekhar, and A. E. White, “Polarization-insensitive planar lightwave circuit dual-rate Mach-Zehnder delay-interferometer,” IEEE Photon. Technol. Lett. **18**(16), 1708–1710 (2006). [CrossRef]

12. L. Eldada and L. W. Shachlette, “Advances in polymer integrated optics,” IEEE J. Sel. Top. Quantum Electron. **6**(1), 54–68 (2000). [CrossRef]

**14**(19), 1339–1365 (2002). [CrossRef]

*L*

_{1}and δ

*L*

_{2}, while the length factors (

*L*+ Δ

*L*)/δ

*L*

_{1}and

*L*/δ

*L*

_{2}are above 10. Thus, the FSR stays within acceptable fluctuations. The fluctuations of FSR are assumed to originate mainly from the environment turbulence. On the other hand, the transmission of the DLI shifts at different heating currents, see the lower row in Fig. 4, because the transmission is dependent on the group refractive index, Eq. (1), which changes at different heating currents. Indeed, the PDFS-free transmission curves, the middle subfigure of the lower row in Fig. 4, can be shifted if waveguide sections on the left and right side of the half-wave plate are heated according to

*T*

_{DLI}– Δ

*T*/2 and

*T*

_{DLI}+ Δ

*T*/2 respectively, as explained in the former section.

19. OIF, “Implementation agreement for integrated dual polarization intradyne coherent receivers,” 2010. http://www.oiforum.com/public/documents/OIF_DPC_RX-01.0.pdf.

## 3. Hybrid integration of photodiode array on polymer PLC

## 4. Adapted common-mode rejection ratio (CMRR) for direct detection receivers

*I*

_{1}and

*I*

_{2}as shown in Fig. 1(a), are going to be amplified differentially, we use an adapted common-mode rejection ratio (CMRR) to characterize this direct detection receiver. Normally, CMRR is a measure of the electrical power balance with respect to the PD responsivity and defined as [19

19. OIF, “Implementation agreement for integrated dual polarization intradyne coherent receivers,” 2010. http://www.oiforum.com/public/documents/OIF_DPC_RX-01.0.pdf.

19. OIF, “Implementation agreement for integrated dual polarization intradyne coherent receivers,” 2010. http://www.oiforum.com/public/documents/OIF_DPC_RX-01.0.pdf.

20. Y. Painchaud, M. Poulin, M. Morin, and M. Têtu, “Performance of balanced detection in a coherent receiver,” Opt. Express **17**(5), 3659–3672 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-5-3659. [CrossRef] [PubMed]

20. Y. Painchaud, M. Poulin, M. Morin, and M. Têtu, “Performance of balanced detection in a coherent receiver,” Opt. Express **17**(5), 3659–3672 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-5-3659. [CrossRef] [PubMed]

20. Y. Painchaud, M. Poulin, M. Morin, and M. Têtu, “Performance of balanced detection in a coherent receiver,” Opt. Express **17**(5), 3659–3672 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-5-3659. [CrossRef] [PubMed]

**17**(5), 3659–3672 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-5-3659. [CrossRef] [PubMed]

*I*

_{1,max}and

*I*

_{1,min}and

*I*

_{2,max}and

*I*

_{2,min}denote the maximum and minimum photo currents of PD1 or PD2, respectively. The phase relation between two minima, e.g.

*I*

_{1}as the reference in Fig. 8, is 2π. In an ideal case, the current response

*I*

_{2}has a relative phase of π to

*I*

_{1}, as shown in Fig. 8(a). The parameters

*I*

_{1,π/2}and

*I*

_{1,3π/2}or

*I*

_{2,π/2}and

*I*

_{2,3π/2}are the current responses of PD1 or PD2 at the wavelengths, which have a phase relation of π/2 or 3π/2 to the reference wavelength point with 0-phase. First, we define the CMRR

_{π/2}and CMRR

_{3π/2}at these two wavelength points as

_{DLI}is the maximum (i.e. the worst value) of the CMRR

_{π/2}and CMRR

_{3π/2},

*I*

_{1,}

*−*

_{φ}*I*

_{2,}

*| for*

_{φ}*φ*= π/2 or 3π/2 are used in the numerator of Eq. (7) rather than |

*I*

_{1, max}−

*I*

_{2, max}| as in [20

**17**(5), 3659–3672 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-5-3659. [CrossRef] [PubMed]

**17**(5), 3659–3672 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-5-3659. [CrossRef] [PubMed]

*I*

_{1,π/2}and

*I*

_{1,3π/2}or

*I*

_{2,π/2}and

*I*

_{2,3π/2}are the half of the value of

*I*

_{1,max}or

*I*

_{2,max}respectively. Also, the difference terms |

*I*

_{1,π/2}−

*I*

_{2,π/2}| and |

*I*

_{1,3π/2}−

*I*

_{2,3π/2}| are the half of the difference term |

*I*

_{1, max}−

*I*

_{2, max}|. Thus, a factor of 2 is added to the terms |

*I*

_{1,}

*−*

_{φ}*I*

_{2,}

*| in Eq. (7). Furthermore, the difference terms |*

_{φ}*I*

_{1(2), max}−

*I*

_{1(2), min}| in the denominator of Eq. (7) are used to take into account not only the unequal responsivities of the PDs and the uneven split of the input power by the DLI, but also the extinction ratio. It can be seen that, in the ideal case without any phase errors and for negligible values of

*I*

_{1,min}and

*I*

_{2,min}, the value of the CMRR

_{DLI}is same as the value of the standard CMRR of Eq. (6).

*φ*, Fig. 8(b), CMRR

_{DLI}is degraded via the change of the difference term |

*I*

_{1,π/2}−

*I*

_{2,π/2}| or |

*I*

_{1,3π/2}−

*I*

_{2,3π/2}| accordingly. In reality, the current response

*I*

_{2}may

*not*be at the ideal (i.e. relative phase of π to

*I*

_{1}) position. If only a phase error is added in Fig. 8(b), the maximum and minimum current values stay the same with respect to those in Fig. 8(a). However, the difference terms |

*I*

_{1,}

*−*

_{φ}*I*

_{2,}

*| change considerably. Comparing Fig. 8(a) and (b), the difference term |*

_{φ}*I*

_{1,π/2}−

*I*

_{2,π/2}| is smaller while |

*I*

_{1,3π/2}−

*I*

_{2,3π/2}| is bigger. This leads to smaller (better) CMRR

_{π/2}and bigger (worse) CMRR

_{3π/2}. In this case, CMRR

_{DLI}takes the maximum value, i.e. CMRR

_{3π/2}, and is degraded by the phase error.

_{DLI}is calculated from Eq. (7) and (8), and plotted as black lines in Fig. 9 . Note that the values of

*I*

_{1,}

*and*

_{φ}*I*

_{2,}

*in Eq. (7) are taken at the wavelengths characterized by the relative phase of π/2 or 3π/2 to the reference wavelength point, while the values of*

_{φ}*I*

_{1,max}and

*I*

_{1,min}or

*I*

_{2,max}and

*I*

_{2,min}at the same wavelengths are obtained by interpolating the neighboring peaks. For comparison, CMRR

_{π/2}and CMRR

_{3π/2}from Eq. (7) and the standard CMRR from Eq. (6) are also plotted as green, red and blue lines in Fig. 9. As expected, the adapted CMRR

_{DLI}is the worst case of the CMRR

_{π/2}and CMRR

_{3π/2}, and degraded by the phase error with respect to the standard CMRR.

_{DLI}shown in Fig. 9 is obtained under DC condition, i.e. with cw input lights. CMRR

_{DLI}versus the modulation frequency can also be measured, if an intensity modulator is available in the optical source as demonstrated in [20

**17**(5), 3659–3672 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-5-3659. [CrossRef] [PubMed]

_{DLI}versus the modulation frequency can be estimated as follows.

- • Firstly, the worst standard CMRR in Fig. 9 is −34 dB. Assuming negligible values of
*I*_{1,min}and*I*_{2,min}, and assuming*I*_{1,max}≈*I*_{2,max}, we have (*I*_{1,max}−*I*_{2,max}) ≈10^{−1.7}⋅(*I*_{1,max}+*I*_{2,max}) ≈10^{−1.7}⋅2⋅*I*_{1,max}≈0.04⋅*I*_{1,max}. - • Secondly, assuming that the worst CMRR
_{DLI}is −20 dB we have (*I*_{1,}−_{φ}*I*_{2,}) ≈10_{φ}^{−1}⋅(*I*_{1,max}+*I*_{2,max}) /2 ≈0.1⋅*I*_{1,max}, for*φ*= π/2 or 3π/2. As discussed in Fig. 8, the half of the (*I*_{1,max}−*I*_{2,max}) is included in (*I*_{1,}−_{φ}*I*_{2,}); thus, the phase error induces an extra difference of 0.08⋅_{φ}*I*_{1,max}in (*I*_{1,}−_{φ}*I*_{2,})._{φ} - • Now, for CMMR
_{DLI}(Ω) at different modulation frequency Ω, the current responses are Î_{1}(Ω) and Î_{2}(Ω) . The values of maxima Î_{1,max}(Ω) and Î_{2,max}(Ω) can be referred to in Fig. 6(b). The largest value of 20⋅log_{10}[Î_{1,max}(Ω) / Î_{2,max}(Ω)] of the PD array from Fig. 6(b) is ~1dB for Ω up to 25 GHz. This gives [Î_{1,max}(Ω) − Î_{2,max}(Ω)] ≈(1−10^{−0.05}) ⋅Î_{1,max}(Ω) ≈0.11⋅Î_{1,max}(Ω). Again, the half of the [Î_{1,max}(Ω) − Î_{2,max}(Ω)] is included in [Î_{1,φ}(Ω) − Î_{2,φ}(Ω)]. Since Î_{1,max}(Ω) does not change considerable with Ω up to 25 GHz, we assume that Î_{1,max}(Ω) ≈*I*_{1,max}and the phase error induces a similar extra difference of 0.08⋅Î_{1,max}(Ω) in [Î_{1,φ}(Ω) − Î_{2,φ}(Ω)]. Then, in the worst case, [Î_{1,φ}(Ω) − Î_{2,φ}(Ω)] is 0.135⋅Î_{1,max}(Ω). This estimation leads to the worst CMRR_{DLI}(Ω) of −17.4 dB, which is above the system requirement under AC condition [19

## 5. Conclusion

## Acknowledgments

## References and links

1. | A. Gnauck and P. Winzer, “Optical phase-shift-keyed transmission,” J. Lightwave Technol. |

2. | H. Kim and P. Winzer, “Robustness to laser frequency offset in direct detection DPSK and DQPSK systems,” J. Lightwave Technol. |

3. | C. R. Doerr, D. M. Gill, A. H. Gnauck, L. I. Buhl, P. J. Winzer, M. A. Cappuzzo, A. Wong-Foy, E. Y. Chen, and L. T. Gomez, “Monolithic demonstrator for 40 Gb/s DQPSK using a star coupler,” J. Lightwave Technol. |

4. | C. R. Doerr, M. A. Cappuzzo, E. Y. Chen, A. Wong-Foy, L. T. Gomez, S. S. Patel, S. Chandrasekhar, and A. E. White, “Polarization-insensitive planar lightwave circuit dual-rate Mach-Zehnder delay-interferometer,” IEEE Photon. Technol. Lett. |

5. | M. Oguma, Y. Nasu, H. Takahashi, H. Kawakami, and E. Yoshida, “Single MZI-based 1×4 DQPSK demodulator,” in Proc. 33rd ECOC (Berlin, Germany, 2007), pp. 147 – 148. |

6. | Y. Nasu, Y. Hashizume, Y. Sakamaki, T. Hashimoto, K. Hattori, and Y. Inoue, “Reduction of Polarization Dependence of PLC Mach-Zehnder Interferometer Over Wide Wavelength Range,” J. Lightw. Technol. |

7. | Y. Nasu, M. Oguma, T. Hashimoto, H. Takahashi, Y. Inoue, H. Kawakami, and E. Yoshida, “Asymmetric Half-Wave Plate Configuration of PLC Mach–Zehnder Interferometer for Polarization Insensitive DQPSK Demodulator,” J. Lightw. Technol. |

8. | J. Gamet and G. Pandraud, “C- and L-Band planar delay interferometer for DPSK decoders,” IEEE Photon. Technol. Lett. |

9. | H. H. Yaffe, C. H. Henry, R. F. Kazarinov, and M. A. Milbrodt, “Polarization-independent silica-on-silicon Mach-Zehnder Interferometers,” J. Lightwave Technol. |

10. | J. Li, K. Worms, D. Hillerkuss, B. Richter, R. Maestle, W. Freude, and J. Leuthold, “Tunable free space optical delay interferometer for demodulation of differential phase shift keying signals”, in Proc. OFC’10 (San Diego, CA, USA, 2010), pp. 1–3. |

11. | N. Keil, C. Zawadzki, Z. Zhang, J. Wang, N. Mettbach, N. Grote, and M. Schell, “Polymer PLC as an Optical Integration Bench,” in Proc. OFC’11 (Los Angeles, CA, USA, 2011), paper OWM1. |

12. | L. Eldada and L. W. Shachlette, “Advances in polymer integrated optics,” IEEE J. Sel. Top. Quantum Electron. |

13. | H. Ma, A. K.-Y. Jen, and L. R. Dalton, “Polymer-based optical waveguides: materials, processing, and devices,” Adv. Mater. (Deerfield Beach Fla.) |

14. | G. Yu, J. Mallari, H. Shen, E. Miller, C. Wei, V. Shofman, D. Jin, B. Chen, H. Chen, and R. Dinu, “40GHz zero chirp single-ended EO polymer modulators with low half-wave voltage,” in Proc. CLEO 2011 (Baltimore, MD, USA, 2011). |

15. | Technical documentations, ZPU12-RI & ZPU13-RI (UV curable polymers), ChemOptics co., Korea. http://inct.raonnet.com/admin_e/pageMake_proto.php?a_name=VGVjaG5vbG9neSBEb2N1bWVudHM=&aa_code=1214. |

16. | N. Keil, H. H. Yao, C. Zawadzki, K. Lösch, K. Satzke, W. Wischmann, J. V. Wirth, J. Schneider, J. Bauer, and M. Bauer, “Hybrid polymer/silica thermo-optic vertical coupler switches,” Appl. Phys. B |

17. | M. Seimetz, “High-order modulation for optical fiber transmission,” in |

18. | M. Schell, N. Keil, H. Yao, and C. Zawadzki, “Method and apparatus for compensating polarization-dependent frequency shifts in optical waveguides,” U.S. Patent 2010/0209, 039, (2010). |

19. | OIF, “Implementation agreement for integrated dual polarization intradyne coherent receivers,” 2010. http://www.oiforum.com/public/documents/OIF_DPC_RX-01.0.pdf. |

20. | Y. Painchaud, M. Poulin, M. Morin, and M. Têtu, “Performance of balanced detection in a coherent receiver,” Opt. Express |

**OCIS Codes**

(130.3120) Integrated optics : Integrated optics devices

(250.5460) Optoelectronics : Polymer waveguides

**ToC Category:**

Integrated Optics

**History**

Original Manuscript: May 19, 2011

Revised Manuscript: June 1, 2011

Manuscript Accepted: June 1, 2011

Published: June 8, 2011

**Citation**

Jin Wang, Crispin Zawadzki, Nelson Mettbach, Walter Brinker, Ziyang Zhang, Detlef Schmidt, Norbert Keil, Norbert Grote, and Martin Schell, "Polarization insensitive 25-Gbaud direct D(Q)PSK receiver based on polymer planar lightwave hybrid integration platform," Opt. Express **19**, 12197-12207 (2011)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-13-12197

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### References

- A. Gnauck and P. Winzer, “Optical phase-shift-keyed transmission,” J. Lightwave Technol. 23(1), 115–130 (2005). [CrossRef]
- H. Kim and P. Winzer, “Robustness to laser frequency offset in direct detection DPSK and DQPSK systems,” J. Lightwave Technol. 21(9), 1887–1891 (2003). [CrossRef]
- C. R. Doerr, D. M. Gill, A. H. Gnauck, L. I. Buhl, P. J. Winzer, M. A. Cappuzzo, A. Wong-Foy, E. Y. Chen, and L. T. Gomez, “Monolithic demonstrator for 40 Gb/s DQPSK using a star coupler,” J. Lightwave Technol. 24(1), 171–174 (2006). [CrossRef]
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