## Experimental study of high sensitivity infrared spectrometer with waveguide-based up-conversion detector^{1}

Optics Express, Vol. 17, Issue 16, pp. 14395-14404 (2009)

http://dx.doi.org/10.1364/OE.17.014395

Acrobat PDF (285 KB)

### Abstract

We have developed an up-conversion spectrometer for signals at single photon levels near the infrared region based on a tunable up-conversion detector that uses a periodically poled lithium niobate waveguide as the conversion medium. We also experimentally studied its characteristics including sensitivity, dark count rate, spectral scan speed, signal transfer function of the waveguide, and polarization sensitivity. The overall single photon detection efficiency of the up-conversion spectrometer is about 32%. With its ultra high sensitivity the spectrometer can measure spectra for signals at a level as low as -126 dBm. We have demonstrated the spectrometers high sensitivity by measuring the spectrum of a greatly attenuated multimode emission from a laser diode at the 1310 nm band.

© 2009 OSA

## 1. Introduction

7. Q. Zhang, C. Langrock, M. M. Fejer, and Y. Yamamoto, “Waveguide-based single-pixel up-conversion infrared spectrometer,” Opt. Express **16**(24), 19557–19561 (2008). [CrossRef] [PubMed]

9. O. Kuzucu, F. N. Wong, S. Kurimura, and S. Tovstonog, “Time-resolved single-photon detection by femtosecond upconversion,” Opt. Lett. **33**(19), 2257–2259 (2008). [CrossRef] [PubMed]

## 2. System configuration

5. H. Xu, L. Ma, A. Mink, B. Hershman, and X. Tang, “1310-nm quantum key distribution system with up-conversion pump wavelength at 1550 nm,” Opt. Express **15**(12), 7247–7260 (2007). [CrossRef] [PubMed]

6. H. Xu, L. Ma, and X. Tang, ““Low noise PPLN-based single photon detector,” Optics East 07,” Proc. SPIE **6780**, 67800U (2007). [CrossRef]

5. H. Xu, L. Ma, A. Mink, B. Hershman, and X. Tang, “1310-nm quantum key distribution system with up-conversion pump wavelength at 1550 nm,” Opt. Express **15**(12), 7247–7260 (2007). [CrossRef] [PubMed]

6. H. Xu, L. Ma, and X. Tang, ““Low noise PPLN-based single photon detector,” Optics East 07,” Proc. SPIE **6780**, 67800U (2007). [CrossRef]

5. H. Xu, L. Ma, A. Mink, B. Hershman, and X. Tang, “1310-nm quantum key distribution system with up-conversion pump wavelength at 1550 nm,” Opt. Express **15**(12), 7247–7260 (2007). [CrossRef] [PubMed]

6. H. Xu, L. Ma, and X. Tang, ““Low noise PPLN-based single photon detector,” Optics East 07,” Proc. SPIE **6780**, 67800U (2007). [CrossRef]

## 3. Performance of up-conversion spectrometer

### 3.1. Sensitivity of an up-conversion spectrometer

*η*is the overall detection efficiency of the up-conversion detector;

_{o}*η*is the total loss in the detector, including the component insertion loss and waveguide coupling loss;

_{loss}*η*is the internal conversion efficiency in the PPLN, that can be estimated according to Eq. (1);

_{con}*η*

_{det}is the detection efficiency of Si-APD at the converted wavelength, which is 710 nm in our case. According to the specification of the Si-APD,

*η*

_{det}is about 65%.

*P*represents the pump power near 1550 nm,

_{pump}*α*is a constant, and

*L*is the length of the waveguide. The measured conversion efficiency vs. pump power is shown in Fig. 2 (a). The maximum overall detection efficiency is 32%, which corresponds to 100% of internal conversion efficiency after we exclude the component loss, waveguide coupling loss, and the detection efficiency of the Si-APD. The dependence of the detection efficiency on the pump power satisfies Eq. (1).

7. Q. Zhang, C. Langrock, M. M. Fejer, and Y. Yamamoto, “Waveguide-based single-pixel up-conversion infrared spectrometer,” Opt. Express **16**(24), 19557–19561 (2008). [CrossRef] [PubMed]

7. Q. Zhang, C. Langrock, M. M. Fejer, and Y. Yamamoto, “Waveguide-based single-pixel up-conversion infrared spectrometer,” Opt. Express **16**(24), 19557–19561 (2008). [CrossRef] [PubMed]

### 3.2. The detection “dead time” and maximum measurement power

*t*) to recover its initial operation state before detection of the next photon. During this period, the bias voltage across the p-n junction of the APD is below the breakdown level and no photon can be detected. This is especially significant when the power of the signal being measured becomes stronger. When the input signal is coherent light and the photon arrival time satisfies a Poisson distribution, the actual count rate can be estimated by [11

_{dead}11. X. Tang, L. Ma, A. Mink, A. Nakassis, H. Xu, B. Hershman, J. C. Bienfang, D. Su, R. F. Boisvert, C. W. Clark, and C. J. Williams, “Experimental study of high speed polarization-coding quantum key distribution with sifted-key rates over Mbit/s,” Opt. Express **14**(6), 2062–2070 (2006). [CrossRef] [PubMed]

*t*is 50 ns for the Si-APD used in the spectrometer. R

_{dead}_{1}is the detection count rate for the Si-APD assuming

*t*is zero, and can be calculated by:

_{dead}*η*is the detection efficiency;

*P*and

_{input}*λ*are the power and wavelength of the signal being measured;

_{input}*ħ*is Plancks constant and c is the speed of light. We used attenuated light from a 1310 nm tunable laser (Santec: TSL-210V) to measure the count rate as a function of the input power. The calculated value is given by Eq. (2) and (3) and the measured values are shown in Fig. 3 (a). When the signal power is lower than -90 dBm, the influence of the dead time on the count rate is small and negligible, but when the power further increases, the influence will be significant. Figure 3 (b) shows the calculated ratio of R/R

_{1}as a function of the signal power. When the signal power is lower than -95 dBm, the ratio is larger than 0.96 and we do not need to consider the influence of the dead-time. When the signal power is between -95 dBm to -80 dBm, the influence of the dead time is significant and the measured spectrum should be calibrated using the ratio curve (in Fig. 3 (b)) to recover the actual spectrum. When the signal power is larger than -80 dBm, more than half of the signal photons are lost due to the dead time and, additionally, the Si-APD is saturated and it is not suitable to use the spectrometer to measure the signal directly. Therefore, the most suitable measurement power range of the spectrometer is from -126 dBm to -95 dBm while the signal between -95 dBm to -80 dBm should calibrated to remove the influence of the dead-time. Any signal above -80 dBm should to be attenuated before using the up-conversion spectrometer.

### 3.3. Waveguide transfer function

^{2}function as given in the following equation [12

12. M. Fejer, G. Magel, D. Jundt, and R. Byer, “Quasi-phase-matched second harmonic generation: tuning and tolerances,” IEEE J. Quantum Electron. **28**(11), 2631–2654 (1992). [CrossRef]

13. M. P. De Micheli, “*χ*^{2} effects in waveguides,” Quantum Semiclassic. Opt. **9**(2), 155–164 (1997). [CrossRef]

*P*,

_{SFG}*P*,

_{pump}*P*are the power of SFG, pump, and signal light,

_{signal}*L*is the waveguide length, and Δ

*k*is the wave-vector-mismatching and can be calculated by the following equation:

*λ*,

_{SFG}*λ*and

_{pump}*λ*are SFG, pump, and signal wavelengths;

_{signal}*n*,

_{SFG}*n*, and

_{pump}*n*are the indices of the nonlinear material for the corresponding wavelength. ∧ is the poling period for the m

_{signal}^{th}order quasi-phase-matched condition of the nonlinear PPLN waveguide.

^{2}function instead of a single peak. It causes some “fake” side peaks in the spectrum measurement when the spectrometer is used to measure a signal with a narrow linewidth. In theory, the two main side peaks are as large as about 5% of the main peak, while in practice, imperfect poling and period uniformity will cause the side peaks to be larger than theoretically predicted and they can also be asymmetric. Because periodic poling at periods of 10–15 microns in congruent lithium niobate is well developed, we believe the main reason for the imperfect transfer function to be the imperfect period uniformity of the waveguide over its 5 cm length.

*S*and

_{measured}*S*are the measured spectrum and the actual signal spectrum.

_{signal}*F*is the transfer function of the waveguide, which can be measured accurately by an optical power meter using strong light from a tunable laser.

*ε*is the total measurement noise, including the dark counts and other measurement noise.

*ε*, is small, we can deconvolve the measured spectrum,

*S*, to recover the actual signal spectrum,

_{measured}*S*. However, the measurement noise is also deconvolved by

_{signal}*F*and then added into the result in the process. The lower the signal-to-noise ratio of the measurement, the worse the estimation of the deconvolved signal. Therefore, to do the recovery by deconvolution, the measurement result must have a sufficiently high signal-to-noise ratio.

14. N. Wiener,* The Extrapolation, Interpolation, and Smoothing of Stationary Time Series with Engineering Applications* (Wiley, 1949) [PubMed]

### 3.4. Spectral resolution and scan speed

^{-6}nm. The tuning step of the pump laser used in the experiment is 0.02 nm (FWHM). The acceptance spectral width for the 5 cm long PPLN waveguide is measured to be 0.2 nm, and dominates the resolution of the up-conversion spectrometer because it is much larger than the spectral bandwidth and tuning resolution of the pump laser. According to Eq. (4) and Eq. (5), the bandwidth of QPM crystal is inversely proportional to the waveguide length

*L*. Therefore, a longer waveguide will result in a better spectral resolution. Due to fabrication tolerances, it is hard to get a PPLN waveguide longer than 5 cm, which is used in this experiment. Therefore, the spectral resolution of an up-conversion spectrometer is limited to about 0.2 nm under current technological conditions. A better spectral resolution can be realized when longer QPM structures are available or other experimental arrangements are implemented.

*v*is the spectrometer scan speed,

_{s}*v*is the tuning speed of pump laser (12 nm/s in our case),

_{t}*n*is the number of steps per nanometer and

*t*is the integration time at each measurement point.

_{in}*n*can be selected according to the desired measurement resolution. In the case of our up-conversion spectrometer, a value of n = 10 (each step = 0.1 nm) is chosen for good resolution given the acceptance spectral width of 0.2 nm.

*t*can be selected by the power of the signal being measured, usually 50~500 ms. In this case, the scan speed of the spectrometer is about 0.2~1 nm/s.

_{in}### 3.5. Polarization sensitivity

^{2}(θ). The high polarization extinction ratio of the PPLN waveguide provides a unique characteristic for up-conversion spectrometers, acting as a polarizer inserted at the front of a traditional spectrometer. Then, the measurement result is the spectrum of the signal at a certain polarization orientation. When the signal being measured is polarized in a certain direction, an up-conversion spectrometer allows us to measure its spectrum and reduce the noise in other polarization orientations. The up-conversion spectrometer is especially suitable for applications in which a polarization spectrum is of interest.

## 4. Experimental result

## 5. Conclusion

## Acknowledgement

## References and links

1. | B. H. Stuart, |

2. | A. Vandevender and P. Kwiat, “High efficiency single photon detection via frequency up-conversion,” J. Mod. Opt. |

3. | C. Langrock, E. Diamanti, R. V. Roussev, Y. Yamamoto, M. M. Fejer, and H. Takesue, “Highly efficient single-photon detection at communication wavelengths by use of upconversion in reverse-proton-exchanged periodically poled LiNbO3 waveguides,” Opt. Lett. |

4. | R. T. Thew, S. Tanzilli, L. Krainer, S. C. Zeller, A. Rochas, I. Rech, S. Cova, H. Zbinden, and N. Gisin, “Low jitter up-conversion detectors for telecom wavelength GHz QKD,” N. J. Phys. |

5. | H. Xu, L. Ma, A. Mink, B. Hershman, and X. Tang, “1310-nm quantum key distribution system with up-conversion pump wavelength at 1550 nm,” Opt. Express |

6. | H. Xu, L. Ma, and X. Tang, ““Low noise PPLN-based single photon detector,” Optics East 07,” Proc. SPIE |

7. | Q. Zhang, C. Langrock, M. M. Fejer, and Y. Yamamoto, “Waveguide-based single-pixel up-conversion infrared spectrometer,” Opt. Express |

8. | M. F. DeCamp and A. Tokmakoff, “Upconversion multichannel infrared spectrometer,” Opt. Lett. |

9. | O. Kuzucu, F. N. Wong, S. Kurimura, and S. Tovstonog, “Time-resolved single-photon detection by femtosecond upconversion,” Opt. Lett. |

10. | http://optoelectronics.perkinelmer.com/catalog/Product.aspx?ProductID=SPCM-AQR-14 |

11. | X. Tang, L. Ma, A. Mink, A. Nakassis, H. Xu, B. Hershman, J. C. Bienfang, D. Su, R. F. Boisvert, C. W. Clark, and C. J. Williams, “Experimental study of high speed polarization-coding quantum key distribution with sifted-key rates over Mbit/s,” Opt. Express |

12. | M. Fejer, G. Magel, D. Jundt, and R. Byer, “Quasi-phase-matched second harmonic generation: tuning and tolerances,” IEEE J. Quantum Electron. |

13. | M. P. De Micheli, “ |

14. | N. Wiener, |

**OCIS Codes**

(190.4410) Nonlinear optics : Nonlinear optics, parametric processes

(230.7370) Optical devices : Waveguides

(300.6340) Spectroscopy : Spectroscopy, infrared

**ToC Category:**

Spectroscopy

**History**

Original Manuscript: June 8, 2009

Revised Manuscript: July 16, 2009

Manuscript Accepted: July 19, 2009

Published: July 31, 2009

**Citation**

Lijun Ma, Oliver Slattery, and Xiao Tang, "Experimental study of high sensitivity infrared spectrometer with waveguide-based up-conversion detector^{1}," Opt. Express **17**, 14395-14404 (2009)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-16-14395

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### References

- B. H. Stuart, Infrared Spectroscopy: Fundamentals and Applications (Wiley, 2004)
- A. Vandevender and P. Kwiat, “High efficiency single photon detection via frequency up-conversion,” J. Mod. Opt. 51, 1433–1445 (2004).
- C. Langrock, E. Diamanti, R. V. Roussev, Y. Yamamoto, M. M. Fejer, and H. Takesue, “Highly efficient single-photon detection at communication wavelengths by use of upconversion in reverse-proton-exchanged periodically poled LiNbO3 waveguides,” Opt. Lett. 30(13), 1725–1727 (2005). [CrossRef] [PubMed]
- R. T. Thew, S. Tanzilli, L. Krainer, S. C. Zeller, A. Rochas, I. Rech, S. Cova, H. Zbinden, and N. Gisin, “Low jitter up-conversion detectors for telecom wavelength GHz QKD,” N. J. Phys. 8(3), 1–12 (2006). [CrossRef]
- H. Xu, L. Ma, A. Mink, B. Hershman, and X. Tang, “1310-nm quantum key distribution system with up-conversion pump wavelength at 1550 nm,” Opt. Express 15(12), 7247–7260 (2007). [CrossRef] [PubMed]
- H. Xu, L. Ma, and X. Tang, ““Low noise PPLN-based single photon detector,” Optics East 07,” Proc. SPIE 6780, 67800U (2007). [CrossRef]
- Q. Zhang, C. Langrock, M. M. Fejer, and Y. Yamamoto, “Waveguide-based single-pixel up-conversion infrared spectrometer,” Opt. Express 16(24), 19557–19561 (2008). [CrossRef] [PubMed]
- M. F. DeCamp and A. Tokmakoff, “Upconversion multichannel infrared spectrometer,” Opt. Lett. 30(14), 1818–1820 (2005). [CrossRef] [PubMed]
- O. Kuzucu, F. N. Wong, S. Kurimura, and S. Tovstonog, “Time-resolved single-photon detection by femtosecond upconversion,” Opt. Lett. 33(19), 2257–2259 (2008). [CrossRef] [PubMed]
- http://optoelectronics.perkinelmer.com/catalog/Product.aspx?ProductID=SPCM-AQR-14
- X. Tang, L. Ma, A. Mink, A. Nakassis, H. Xu, B. Hershman, J. C. Bienfang, D. Su, R. F. Boisvert, C. W. Clark, and C. J. Williams, “Experimental study of high speed polarization-coding quantum key distribution with sifted-key rates over Mbit/s,” Opt. Express 14(6), 2062–2070 (2006). [CrossRef] [PubMed]
- M. Fejer, G. Magel, D. Jundt, and R. Byer, “Quasi-phase-matched second harmonic generation: tuning and tolerances,” IEEE J. Quantum Electron. 28(11), 2631–2654 (1992). [CrossRef]
- M. P. De Micheli, “χ2 effects in waveguides,” Quantum Semiclassic. Opt. 9(2), 155–164 (1997). [CrossRef]
- N. Wiener, The Extrapolation, Interpolation, and Smoothing of Stationary Time Series with Engineering Applications (Wiley, 1949) [PubMed]

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