## Free-space optical delay interferometer with tunable delay and phase |

Optics Express, Vol. 19, Issue 12, pp. 11654-11666 (2011)

http://dx.doi.org/10.1364/OE.19.011654

Acrobat PDF (2073 KB)

### Abstract

A free-space optical delay interferometer (DI) featuring a continuously tunable time delay, polarization insensitive operation with high extinction ratios and accurate phase and time delay monitoring scheme is reported. The polarization dependence is actively mitigated by adjusting a birefringent liquid-crystal device. The DI has been tested for reception of D(m)PSK signals.

© 2011 OSA

## 1. Introduction

1. R. A. Griffin, R. I. Johnstone, R. G. Walker, J. Hall, S. D. Wadsworth, K. Berry, A. C. Carter, M. J. Wale, J. Hughes, P. A. Jerram, and N. J. Parsons, “10 Gb/s optical differential quadrature phase shift key (DQPSK) transmission using GaAs/AlGaAs integration,” in *Proc. Optical Fiber Communication Conference (OFC'02)*, (Anaheim, CA, USA, 2002), Postdeadline Paper FD6.

2. A. H. Gnauck, G. Raybon, S. Chandrasekhar, J. Leuthold, C. Doerr, L. Stulz, A. Agarwal, S. Banerjee, D. Grosz, S. Hunsche, A. Kung, A. Marhelyuk, D. Maywar, M. Movassaghi, X. Liu, C. Xu, X. Wei, and D. M. Gill, “2.5 Tb/s (64´42.7 Gb/s) transmission over 40´100 km NZDSF using RZ-DPSK format and all-Raman-amplified spans,” in *Proc. Optical Fiber Communication Conference (OFC'02)*, (Anaheim, CA, USA, 2002), Postdeadline Paper PD FC2.

4. X. Liu, S. Chandrasekhar, and A. Leven, “Digital self-coherent detection,” Opt. Express **16**(2), 792–803 (2008). [CrossRef] [PubMed]

6. J. Leuthold, B. Mikkelsen, G. Raybon, C. H. Joyner, J. L. Pleumeekers, B. I. Miller, K. Dreyer, and R. Behringer, “All-optical wavelength conversion between 10 and 100 Gb/s with SOA delayed-interference configuration,” Opt. Quantum Electron. **33**(7/10), 939–952 (2001). [CrossRef]

7. S. Sygletos, R. Bonk, T. Vallaitis, A. Marculescu, P. Vorreau, J. Li, R. Brenot, F. Lelarge, G.-H. Duan, W. Freude, and J. Leuthold, “Filter assisted wavelength conversion with quantum-dot SOAs,” J. Lightwave Technol. **28**(6), 882–897 (2010). [CrossRef]

8. D. Hillerkuss, M. Winter, M. Teschke, A. Marculescu, J. Li, G. Sigurdsson, K. Worms, S. Ben Ezra, N. Narkiss, W. Freude, and J. Leuthold, “Simple all-optical FFT scheme enabling Tbit/s real-time signal processing,” Opt. Express **18**(9Issue 9), 9324–9340 (2010). [CrossRef] [PubMed]

- • Tunability in delay time and tunability for phase. In almost any of the above mentioned applications a good reception quality, requires adaption of the time delay to the symbol rate [9]. In again other instances, it is sometimes advantageous to deviate from the one-symbol delay in order to mitigate transmission impairments caused by effects such bandwidth-narrowing by concatenated filters [10
9. G. Bosco and P. Poggiolini, “On the joint effect of receiver impairments on direct-detection DQPSK systems,” J. Lightwave Technol.

**24**(3), 1323–1333 (2006). [CrossRef]]. A DI with a continuously tunable delay would fulfill both requirements. Recently, a DI with adaptive delay has been presented by using cascaded Mach-Zehnder interferometers [11], however this only provides a discrete switching of three delays. A continuously tunable DI has been proposed in [1210. B. Mikkelsen, C. Rasmussen, P. Mamyshev, and F. Liu, “Partial DPSK with excellent filter tolerance and OSNR sensitivity,” Electron. Lett.

**42**(23), 1363–1364 (2006). [CrossRef]], where the delay is introduced by passing two orthogonal polarizations through a tunable differential group delay (DGD) element. This scheme, however, requires that the input signal has equal power for both orthogonal polarizations.12. L. Christen, Y. K. Lize, S. Nuccio, L. Paraschis, and A. E. Willner, “Experimental demonstration of reduced complexity 43-Gb/s RZ-DQPSK rate-tunable receiver,” IEEE Photon. Technol. Lett.

**20**(13), 1166–1168 (2008). [CrossRef] - • Low polarization-dependent loss (PDL), and especially a low polarization-dependent frequency shift (PDFS), which is of particular importance for demodulating D(m)PSK signals [13]. This is normally achieved by carefully selecting the optical coatings, which should perform for all polarizations alike.
13. H. Kawakami, E. Yoshida, Y. Miyamoto, and M. Oguma, “Analysing the penalty induced by PD of MZI in DQPSK receiver using novel measuring technique,” Electron. Lett.

**43**(2), 121–122 (2007). [CrossRef] - • Accurate monitoring and control of the operating point defining phase and time delay. Phase monitoring with a fixed time delay has been intensively discussed based on RF power monitoring [14] or based on a correlation method [15]. Although the efficiency of both methods has been demonstrated, the techniques are complicated, and they are limited to certain data formats and bitrates. A pilot-tone driven lock-in algorithm can be much more effective in fixing the DI operating point, especially for DQPSK reception, when a DI pair needs be locked to the desired π/2 relative phase offset. Sophisticated time delay control techniques can be found in the field of distance metrology [16
14. H. Kawakami, E. Yoshida, Y. Miyamoto, M. Oguma, and T. Itoh, “Simple phase offset monitoring technique for 43 Gbit/s optical DQPSK receiver,” Electron. Lett.

**44**(6), 437–438 (2008). [CrossRef],17]. A simple solution would be to use a known pilot tone, and to count fringes when tuning the DI.16. L. Rovati, U. Minoni, M. Bonardi, and F. Docchio, “Absolute distance measurement using comb-spectrum interferometry,” J. Opt.

**29**(3), 121–127 (1998). [CrossRef]

## 2. DI modeling

*T*compared to the lower path. The input electrical fields

*E*

_{in,1}and

*E*

_{in,2}are split by the first coupler (

*S*), then experience two different optical paths, and are then combined at the second coupler (

_{I}*S*), where two output fields

_{II}*E*

_{out,1}and

*E*

_{out,2}are generated.

6. J. Leuthold, B. Mikkelsen, G. Raybon, C. H. Joyner, J. L. Pleumeekers, B. I. Miller, K. Dreyer, and R. Behringer, “All-optical wavelength conversion between 10 and 100 Gb/s with SOA delayed-interference configuration,” Opt. Quantum Electron. **33**(7/10), 939–952 (2001). [CrossRef]

*E*

_{in,1,2}, and find the time-dependent output electric fields

*E*

_{out,1,2}(only one polarization is considered):

*S*and

_{I}*S*in Eq. (1) describe the two couplers, where a phase factor j provides the ideal phase relations between the two output signals, which might be impaired by phase offsets

_{II}*δθ*

^{I}_{12},

*δθ*

^{I}_{21},

*δθ*

^{II}_{12}, and

*δθ*

^{II}_{21}.The quantities

*as*,

_{I}*a*(1

*− s*),

_{I}*bs*and

_{II}*b*(1

*− s*) are the power splitting ratios of the two couplers in the form of amplitude factors. If

_{II}*a*= 1 and

*b*= 1, the couplers are lossless. The Dirac distributions in matrix

*T*are the impulse responses of the “long” (upper) and the “short” (lower) arms having a group delay difference Δ

*T*, see Fig. 1. Assuming a monochromatic optical signal with angular frequency

*ω*= 2π

_{c}*f*, a change of Δ

_{c}*T*introduces a phase offset

*−*Δ

*Tω*in the upper arm. The quantities

_{c}*A*,

*B*< 1 are the power loss factors in the two paths.

*E*

_{in,1}≠ 0,

*E*

_{in,2}= 0, Eq. (1) simplifies to

*E*

_{out,1, 2}(

*t*) and

*E*

_{in,1}(

*t*) by

*Ê*

_{out,1,2}(

*f*) and

*Ê*

_{in,1}(

*f*), respectively. The associated transfer functions are

*H*

_{1, 2}(

*f*) = |

*H*

_{1, 2}(

*f*)|exp[ jΦ

_{1, 2}(

*f*)] =

*Ê*

_{out,1, 2}(

*f*) /

*Ê*

_{in,1}(

*f*).

*τ*

_{1, 2}(

*f*) = −1/(2π) dΦ

_{1, 2}(

*f*) / d(

*f*),

*s*,

_{I}*s*= 0,

_{II}*a, b*= 1, the input field

*E*

_{in,1}in Fig. 1 passes exclusively through the lower arm to the output 1. The squared magnitude of the transfer function at the outputs is calculated to be |

*H*

_{1}(

*f*)|

^{2}=

*B*, |

*H*

_{2}(

*f*)|

^{2}= 0, and the output group delay is

*τ*

_{1}(

*f*) = 0 according to the assumption Eq. (1). When

*s*,

_{I}*s*= 1,

_{II}*a, b*= 1, the input

*E*

_{in,1}in Fig. 1 propagates through the upper arm to output 1 without coupling into the lower arm. The squared magnitude of the transfer function at the outputs can be calculated, |

*H*

_{1}(

*f*)|

^{2}=

*A*, |

*H*

_{2}(

*f*)|

^{2}= 0, and the associated output group delay is

*τ*

_{1}(

*f*) = Δ

*T*according to the assumption formulated in Eq. (1).

*f*

_{0}as a frequency where destructive interference occurs at output 1, and constructive interference at output 2. We plot |

*H*

_{1,2}(

*f*)|

^{2}as well as phases Φ

_{1, 2}(

*f*) and group delays

*τ*

_{1, 2}(

*f*) as a function of the frequency offset from

*f*

_{0}, blue curves in Fig. 2 . The frequency offset is given in units of the free spectral range FSR = 1 / Δ

*T*. The two outputs have complementary power transfer functions

*H*

_{1,2}(

*f*)| crosses zero, a π-phase shift is observed, Fig. 2(b). These phase jumps result in δ-functions of the group delay,

*N*= 0, ± 1, ± 2, ± 3,…, while

*τ*

_{1, 2}(

*f*) = Δ

*T*/ 2 holds at all other frequencies, Fig. 2(c).

*A*= 0.64, while all other parameters remain as in Eq. (6). For this case, the quantities |

*H*

_{1,2}(

*f*)|

^{2}, Φ

_{1, 2}(

*f*) and

*τ*

_{1, 2}(

*f*) are plotted in Fig. 2 (red curves). In contrast to the ideal case, the red curves have a finite ER = 20 dB, and an additional loss of 0.085 dB. Furthermore, instead of the π-phase jumps, the phase slopes are reduced near the frequency for destructive interference, which results in a finite negative group delay, Fig. 2(c). This behavior is typical for any deviation of

*A*,

*B*,

*a*,

*b*, and

*s*,

_{I}*s*from the ideal conditions defined in Eq. (6). Imperfect phase offsets

_{II}## 3. DI implementation and polarization dependence

*T*. However we notice that the upper, longer path usually has more loss than the lower path due to the additional reflector, so that the extinction ratio is degraded. Moreover, the minimum time delay difference Δ

*T*between the two paths is also limited by the fact that the upper optical path is always longer.

^{3}can be further decreased by using the LIGA technology as in [18]. The actuator used in this setup has a mechanical step size of about 2 nm, which corresponds to a phase offset of about 1°. The mechanical tuning range amounts to 15 mm corresponding to a time delay tuning range of 100 ps. As the actuator responds within milliseconds, the tuning can be performed extremely fast.

*v*and

*h*(“

*v*” vertical polarization, and “

*h*” horizontal polarization with respect to the incident plane). Because each surface in the system has polarization-dependent transmittivity and reflectivity, the transfer function of the DI is seriously affected by the input signal polarization state. While polarization dependent reflectivity can be minimized by carefully selected coatings, there remains a polarization-dependent phase shift. This phase offset results in a polarization dependent frequency shift (PDFS) of the DI transfer function.

*φ*

_{pol}= 2π (PDFS / FSR) against the voltage applied to the liquid crystal. The resolution of the plot is limited by the resolution of the equipment that allowed us to sweep the wavelength with steps of 10 pm only. For a FSR of 200 GHz (1.6 nm) we can resolve a phase difference of 2.2° (0.61% of FSR).

*T*as an additional free space does not add to the birefringence. In Fig. 4(a), 4(b) one can notice that the ER = (maximum of output 1, 2) / (minimum of output 1, 2) at “Output 2” for both polarizations is larger than 30 dB. At “Output 1” and for

*h*-polarization the ER is 30 dB as well, however for the

*v*-polarization only 18 dB is measured. This difference is due to the fact transmittivities and reflectivities of the BS coating are slightly polarization dependent. In our case output 2 has an almost ideal extinction ratio because the two beams constructively interfering in the beam splitter and being mapped onto output 2 undergo one reflection and one transmission each. However, the two beams interfering in the BS and being mapped to output 1 undergo two reflections and two transmissions, respectively, which leads to a power imbalance (splitting ratio difference) for the slightest imbalance in the transmission and reflection coefficients of the BS coating. As we discussed above, this will lead to the ER difference between the two outputs at two orthogonal polarizations. As commercially available fixed-delay DIs have typically ER > 18 dB, this prototype has comparable performance. A PDFS < (0.61% of FSR) is also good enough for DQPSK demodulation [9

9. G. Bosco and P. Poggiolini, “On the joint effect of receiver impairments on direct-detection DQPSK systems,” J. Lightwave Technol. **24**(3), 1323–1333 (2006). [CrossRef]

13. H. Kawakami, E. Yoshida, Y. Miyamoto, and M. Oguma, “Analysing the penalty induced by PD of MZI in DQPSK receiver using novel measuring technique,” Electron. Lett. **43**(2), 121–122 (2007). [CrossRef]

## 3. Time delay control

*f*counter-propagates with respect to the communication signal at

_{p}*f*. Circulators are used for separation of the pilot and signal tone. Because

_{c}*f*and

_{p}*f*can be widely different, appropriate filters would also allow a co-propagating arrangement without circulators [20

_{c}20. D. O. Caplan, M. L. Stevens, and J. J. Carney, “High-sensitivity multi-channel single-interferometer DPSK receiver,” Opt. Express **14**(23), 10984–10989 (2006). [CrossRef] [PubMed]

*m*

_{max}and

*m*

_{min}, respectively, when adjusting the delay. The associated change

*δT*of the time delay Δ

*T*can be derived by counting the number of minima and maxima that a signal undergoes when detuning the delay

*T*, and Δ

*T*× c (c is the speed of light) is expressed as multiple of the pilot tone wavelength. The remaining delay in distance is less than the wavelength of the pilot tone, which effectively introduces a phase offset to the signal. The remaining phase delay can be found by measuring the power swing of the pilot tone while scanning the delay, and by evaluating the pilot tone power deviation from the average power value at the end of the actuator travel.

20. D. O. Caplan, M. L. Stevens, and J. J. Carney, “High-sensitivity multi-channel single-interferometer DPSK receiver,” Opt. Express **14**(23), 10984–10989 (2006). [CrossRef] [PubMed]

21. A. A. Freschi and J. Frejlich, “Adjustable phase control in stabilized interferometry,” Opt. Lett. **20**(6), 635–637 (1995). [CrossRef] [PubMed]

*f*harmonically according to

_{p}*f*(

*t*) =

*f*+

_{p}*δν*sin (2π

_{d}*ν*

_{FM}

*t*) using a frequency deviation

*δν*with a modulation frequency

_{d}*ν*

_{FM}. Figure 6(b) shows the power transfer function and denotes its FSR, the DI’s operating frequency

*f*

_{0}, and the signal carrier frequency

*f*. Assuming that the pilot tone is connected to output 2 with

_{c}*f*

_{0}being the frequency with constructive interference in the output, the frequency offset between pilot tone and DI is denoted Δ

*f*=

_{p}*f*−

_{p}*f*

_{0}.

_{1,2}stand for the Bessel functions of order 1 and 2. The amplitudes of sin(2π

*ν*

_{FM}t) and cos(4π

*ν*

_{FM}t) can be extracted with a numerical lock-in scheme. Δ

*T*is known with the fringe-counting measurement. Therefore the terms J

_{1,2}(2π

*δν*Δ

_{d}*T*) can be estimated with good accuracy, and we calculate Δ

*f*from the remaining trigonometric terms sin(2πΔ

_{p}*f*Δ

_{p}*T*) and cos(2πΔ

*f*Δ

_{p}*T*). With Δ

*f*and pilot tone

_{p}*f*known, the frequency

_{p}*f*

_{0}of the transfer function’s maximum is found, which has an offset Δ

*f*from the carrier frequency

_{c}*f*. Therefore, based on the known offset Δ

_{c}*f*, a control loop can be set up to fine-tune the delay Δ

_{c}*T*for the wanted offset Δ

*f*, which fixes the phase difference in both arms of the DI. Using the same pilot tone, two DIs in a IQ demodulator can be locked with a defined relative phase offset, i. e. π/2.

_{c}## 4. Measurement and experiment result

*ν*

_{FM}= 60 Hz and

*δν*= 2 GHz and used to scan the phase offset over 360° at FSR = 40 GHz. Results at FSR = 80 GHz are also shown. In Fig. 9(a), the deviations between the set phase offset and the cross-examinations with respect to an OSA are shown. A maximum error of ~2° is found. A 6 hours phase deviation measurement is performed with the pilot tone being modulated with

_{d}*ν*

_{FM}= 30 Hz and

*δν*= 750 MHz. A phase deviation < 2° and a standard deviation of 0.3° has been measured. The result is shown in Fig. 9(c).

_{d}^{7}- 1 at symbol-rates of 28 GBd and 42.7 GBd as depicted in Fig. 10(b). For the two orthogonal polarizations the required received power for fixed BER is almost equal for both symbol-rates with the proposed DI. The performance is comparable with a commercial available DI in the same measurement setup. In Fig. 10(c), similar received power requirement for fixed BER is observed over a broad wavelength range.

## 6. Polarization division differential detector

*T*that is optimized with respect to the symbol rate of the detected signal. The signal is first split into the two polarization components

*E*and

_{x}*E*(which in general do not correspond to the transmitter’s polarization components) by a polarization beam splitter (PBS). Then each polarization is fed into 2 orthogonal DIs (π/2 relative phase offset) (I- and Q-DIs) and four balanced detectors are used to detect the signal components.

_{y}*E*and

_{x}*E*. Next we would like to determine the I- and Q-phase component of each of the polarizations. So we use quarter-wave plates (QWP) to convert the

_{y}*E*and

_{x}*E*components into circular polarizations (which provides us horizontal and vertical fields

_{y}*E*

_{h}and

*E*

_{v}. Please note that the real advantage of this scheme is that we have

*E*

_{h}and

*E*

_{v}fields of equal power!). Each circular polarization is then fed into a separate DI. One branch of the DI comprises the time delay and the other branch a birefringent element (e.g. a liquid crystal Δ

*φ*

_{Pol}). The delay may then be set for the

*h*-component to provide the I-phase component. The birefringent element may then be used to set the phase difference in the short branch to δ

*φ*

_{pol}+ π/2 resulting in a π/2 relative phase offset for the

*v*-component with respect to the

*h*-component. The

*v*-component will thus then provide us the Q-phase term. Using PBSs at the DI outputs the

*E*

_{h}and

*E*

_{v}fields can easily be separated and combined into the balanced detectors. Thus we have reduced the number of required DIs to only 2 DIs in this newly proposed configuration.

*T.*In the other branch, the birefringent element is set to align the orthogonal relative phase offset between the I and Q-components. Two PBSs (including the one at the input) are used to separate the signals into

*h*and

*v*polarizations. The signals can be coupled into fibers or directly to photodiodes providing electrical signals for further processing. With this novel setup, four logic DIs in a polarization diversity self-coherent detection scheme are folded into one single and compact Michelson delay interferometer structure with one single actuator tuning the time delay.

## 6. Conclusion

## Acknowledgments

*EuroFOS*and Karlsruhe School of Optics and Photonics (KSOP).

## References and links

1. | R. A. Griffin, R. I. Johnstone, R. G. Walker, J. Hall, S. D. Wadsworth, K. Berry, A. C. Carter, M. J. Wale, J. Hughes, P. A. Jerram, and N. J. Parsons, “10 Gb/s optical differential quadrature phase shift key (DQPSK) transmission using GaAs/AlGaAs integration,” in |

2. | A. H. Gnauck, G. Raybon, S. Chandrasekhar, J. Leuthold, C. Doerr, L. Stulz, A. Agarwal, S. Banerjee, D. Grosz, S. Hunsche, A. Kung, A. Marhelyuk, D. Maywar, M. Movassaghi, X. Liu, C. Xu, X. Wei, and D. M. Gill, “2.5 Tb/s (64´42.7 Gb/s) transmission over 40´100 km NZDSF using RZ-DPSK format and all-Raman-amplified spans,” in |

3. | N. Kikuchi, K. Mandai, S. Sasaki, and K. Sekine, “Proposal and first experimental demonstration of digital incoherent optical field detector for chromatic dispersion compensation,” in |

4. | X. Liu, S. Chandrasekhar, and A. Leven, “Digital self-coherent detection,” Opt. Express |

5. | J. Li, K. Worms, P. Vorreau, D. Hillerkuss, A. Ludwig, R. Maestle, S. Schuele, U. Hollenbach, J. Mohr, W. Freude, and J. Leuthold, “Optical vector signal analyzer based on differential direct detection,” in |

6. | J. Leuthold, B. Mikkelsen, G. Raybon, C. H. Joyner, J. L. Pleumeekers, B. I. Miller, K. Dreyer, and R. Behringer, “All-optical wavelength conversion between 10 and 100 Gb/s with SOA delayed-interference configuration,” Opt. Quantum Electron. |

7. | S. Sygletos, R. Bonk, T. Vallaitis, A. Marculescu, P. Vorreau, J. Li, R. Brenot, F. Lelarge, G.-H. Duan, W. Freude, and J. Leuthold, “Filter assisted wavelength conversion with quantum-dot SOAs,” J. Lightwave Technol. |

8. | D. Hillerkuss, M. Winter, M. Teschke, A. Marculescu, J. Li, G. Sigurdsson, K. Worms, S. Ben Ezra, N. Narkiss, W. Freude, and J. Leuthold, “Simple all-optical FFT scheme enabling Tbit/s real-time signal processing,” Opt. Express |

9. | G. Bosco and P. Poggiolini, “On the joint effect of receiver impairments on direct-detection DQPSK systems,” J. Lightwave Technol. |

10. | B. Mikkelsen, C. Rasmussen, P. Mamyshev, and F. Liu, “Partial DPSK with excellent filter tolerance and OSNR sensitivity,” Electron. Lett. |

11. | Y. Nasu, K. Hattori, T. Saida, Y. Hashizume, and Y. Sakamaki, “Silica-based adaptive-delay DPSK demodulator with a cascaded Mach-Zehnder interferometer configuration,” in |

12. | L. Christen, Y. K. Lize, S. Nuccio, L. Paraschis, and A. E. Willner, “Experimental demonstration of reduced complexity 43-Gb/s RZ-DQPSK rate-tunable receiver,” IEEE Photon. Technol. Lett. |

13. | H. Kawakami, E. Yoshida, Y. Miyamoto, and M. Oguma, “Analysing the penalty induced by PD of MZI in DQPSK receiver using novel measuring technique,” Electron. Lett. |

14. | H. Kawakami, E. Yoshida, Y. Miyamoto, M. Oguma, and T. Itoh, “Simple phase offset monitoring technique for 43 Gbit/s optical DQPSK receiver,” Electron. Lett. |

15. | Z. Tao, A. Isomura, T. Hoshida, and J. C. Rasmussen, “Dither-free, accurate, and robust phase offset monitor and control method for optical DQPSK demodulator” in |

16. | L. Rovati, U. Minoni, M. Bonardi, and F. Docchio, “Absolute distance measurement using comb-spectrum interferometry,” J. Opt. |

17. | A. Cabral and J. Rebordao, ““Accuracy of frequency-sweeping interferometry for absolute distance metrology,” Opt. Eng. |

18. | S. Schüle, U. Hollenbach, J. Mohr, J. Li, P. Vorreau, A. Efremov, J. Leuthold, and S. Schonhardt, “Modular integration of microactuators and micro-optical benches,” in |

19. | 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 |

20. | D. O. Caplan, M. L. Stevens, and J. J. Carney, “High-sensitivity multi-channel single-interferometer DPSK receiver,” Opt. Express |

21. | A. A. Freschi and J. Frejlich, “Adjustable phase control in stabilized interferometry,” Opt. Lett. |

**OCIS Codes**

(060.5060) Fiber optics and optical communications : Phase modulation

(120.3180) Instrumentation, measurement, and metrology : Interferometry

**ToC Category:**

Instrumentation, Measurement, and Metrology

**History**

Original Manuscript: February 18, 2011

Manuscript Accepted: May 26, 2011

Published: June 1, 2011

**Citation**

Jingshi Li, Kai Worms, Ruediger Maestle, David Hillerkuss, Wolfgang Freude, and Juerg Leuthold, "Free-space optical delay interferometer with tunable delay and phase," Opt. Express **19**, 11654-11666 (2011)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-12-11654

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### References

- R. A. Griffin, R. I. Johnstone, R. G. Walker, J. Hall, S. D. Wadsworth, K. Berry, A. C. Carter, M. J. Wale, J. Hughes, P. A. Jerram, and N. J. Parsons, “10 Gb/s optical differential quadrature phase shift key (DQPSK) transmission using GaAs/AlGaAs integration,” in Proc. Optical Fiber Communication Conference (OFC'02), (Anaheim, CA, USA, 2002), Postdeadline Paper FD6.
- A. H. Gnauck, G. Raybon, S. Chandrasekhar, J. Leuthold, C. Doerr, L. Stulz, A. Agarwal, S. Banerjee, D. Grosz, S. Hunsche, A. Kung, A. Marhelyuk, D. Maywar, M. Movassaghi, X. Liu, C. Xu, X. Wei, and D. M. Gill, “2.5 Tb/s (64´42.7 Gb/s) transmission over 40´100 km NZDSF using RZ-DPSK format and all-Raman-amplified spans,” in Proc. Optical Fiber Communication Conference (OFC'02), (Anaheim, CA, USA, 2002), Postdeadline Paper PD FC2.
- N. Kikuchi, K. Mandai, S. Sasaki, and K. Sekine, “Proposal and first experimental demonstration of digital incoherent optical field detector for chromatic dispersion compensation,” in Proc. of European Conference on Optical Communication (ECOC’06), (Cannes, France, 2006), paper Th444.
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