Periodically poled lithium niobate (PPLN), especially the one fabricated into waveguide, has a conversion efficiency 4 orders of magnitude more than that obtained employing bulk crystals [113, 114].
As shown in Fig. 6.3, the telecommunication wavelength was divided into telecom O, E, S, C, L bands, corresponding to the original-band, extended-band, short-band, conventional-band and long-band. The transmission loss of different bands in optical fiber vary between 0.2 dB/km to 0.8 dB/km. In general, longer wavelengths show lower losses in transmission. At telecom E band, the transmission loss is higher due to the OH− absorption. The minimum loss in this band is about 1 dB/km.
108 DIFFERENCE FREQUENCY GENERATION
1260 nm 1360 nm 1460 nm 1530 nm 1565 nm 1625 nm
Original-band
Wavelength
Extended-band Short-band
Conventional-band
Long-band
O E S C L
Figure 6.3: Definition of different bands in telecommunication wavelength.
Figure 6.4: Measured attenuation in silica fibers (solid line and theoretical limits (dashed lines) given by Rayleigh scattering in the short-wavelength region and by molecular vi-brations (infrared absorption) in the infrared region, adapted from Fig.22.2 of [115].
6.4. DIFFERENCE FREQUENCY GENERATION WITH PPLN 109
Figure 6.5: Conversion efficiency as a function of ω1 andω2. ω1 andω2are the wavelength of signal and pump respectively with 50 mm long and 11 µm period PPLN.
636 636.5 637 637.5 638
Figure 6.6: The oscillation of conversion efficiencyηwith the signal frequency when pump wavelength is fixed at 1040 nm.
6.4.1 DFG based on NV color center
To convert single photon signal from NV color center which has ZPL at 637 nm to telecom-munication frequency, the pump wavelength can be chosen at 1040 nm to get 1550 nm converted light at telecommunication C band (0.2 dB/km).Under room temperature, the suitable period calculated for PPLN crystal is about 11 µm.
In Fig. 6.5, we show the conversion efficiency as a function of signal wavelength and pump wavelength considering a PPLN with 50 mm length and 11 µm period for poling.
When the signal wavelength shifts from 636 nm to 638 nm, the wavelength of the pump meeting the quasi phase matching condition varies from 1030 nm to 1050 nm. For a pump with fixed wavelength, the convertible signal wavelength is quite narrow. The Fig.
6.6 shows the oscillation of conversion efficiency η with the signal frequency when pump wavelength is fixed at 1040 nm. Only the photons between 637±0.1 nm are able to be converted with high efficiency, while the other wavelengths will not be able to join this frequency conversion process. Considering the emission spectrum of NV, a lot of energy will not be converted.
110 DIFFERENCE FREQUENCY GENERATION
Wavelength of pump (nm) pumpidler signal Idler wavelength (nm)
Figure 6.7: (a) Conversion efficiency as a function of the wavelength of signal (ω1) and pump (ω2) respectively with 50 mm long and 16.85 µm period PPLN. In practice, the PPLN period can be finely adjusted with temperature. (b) Phase matching described with ∆k as a function ofω1 andω2. (c) Wavelength of idler light (ω3) as a function of ω1 and ω2.
6.4.2 DFG based on SiV color center
We did the conversion efficiency calculation based on the SiV color center in bulk diamond, whose emission spectrum has relatively narrow linewidth, which is about 2 nm.
To convert single photon signal from SiV color center which has ZPL at 738 nm to telecommunication frequency, the pump wavelength can be chosen at 1403 nm so that we can get 1560 nm converted light at telecommunication C band.Under room temperature, the suitable period calculated for PPLN crystal is 16.8 µm.
In Fig. 6.7-a, we show the conversion efficiency as a function of signal wavelength centered at 738.5 nm and pump wavelength centered at 1400 nm considering a PPLN with 50 mm length and 16.85µm period. The phase matching is described with∆k as a function of ω1 and ω2, shown in Fig. 6.7-b. And the wavelength of converted light as a function of ω1 and ω2 is shown in Fig. 6.7-c.
If we use 1506 nm as pump wavelength, we can convert the 738.5 nm signal to telecom E band instead of telecom C band with slightly higher transmission loss of about 1 dB/km.
In Fig. 6.8-a, we show the conversion efficiency as a function of signal wavelength centered at 738.5 nm and pump wavelength centered at 1560 nm considering a PPLN with 50 mm
6.4. DIFFERENCE FREQUENCY GENERATION WITH PPLN 111
737.5 738 738.5 739 739.5 1540
1550
1560
1570
1580 1385
1395 1405 1415 1425 737.5 738 738.5 739 739.5
Wavelength of signal (nm) 1540
1550
1560
1570
1580
Wavelength of pump (nm)
0 0.2 0.4 0.6 0.8 1
Idler wavelength (nm)
739.5 737.5 738 738.5 739
Wavelength of signal (nm) 1540
1550
1560
1570
1580
Wavelength of pump (nm)
(a)
(b) (c)
pump idler signal
Wavelength of signal (nm)
Wavelength of pump (nm)
Figure 6.8: (a) Conversion efficiency as a function of the wavelength of signal (ω1) and pump (ω2) respectively with 50 mm long and 16.85µm period PPLN. (b) Phase matching described with ∆k as a function of ω1 and ω2. (c) Wavelength of idler light (ω3) as a function of ω1 and ω2.
112 DIFFERENCE FREQUENCY GENERATION
738 738.5 739 738 738.5 739
Figure 6.9: Conversion efficiency when converting signal from SiV color center to 1400 nm (telecom E band) using a pump with a width ∆vp = 1,740±50GHz FWHM varying from 2.5 nm to 10 nm centered at 1560 nm. We use experimentally measured fluorescence spectrum of single SiV color center in nanodiamond at room temperature.
length and 16.85 µm period. The phase matching is described with ∆k as a function of ω1 and ω2, shown in Fig. 6.8-b. And the wavelength of converted light as a function of ω1 and ω2 is shown in Fig. 6.8-c.
We analysis the conversion efficiency of fluorescence from single SiV color center con-verted to 1400 nm at telecom E band and its dependence on the pump linewidth. We calculated the conversion efficiency using the experimentally measured fluorescence spec-trum of a single SiV color center in nanodiamond at room temperature. The specspec-trum linewidth of the SiV color center in nanodiamond we measured is about 5 nm centered at 738.5 nm instead of 2 nm centered at 738 nm. The optical coherence property of defects in nanodiamonds is worse than that in bulk diamond, causing the spectral diffusion of ZPL (or spectral jumping of ZPL frequency) [116]. When we vary the spectrum full width at half maximum (FWHM) of the pump from 2.5 nm to 5 nm and 10 nm centered at 1560 nm (308 GHz, 616 GHz, 1 THz in frequency), the map of conversion efficiency as a function of pump wavelength and signal wavelength is shown in Fig. 6.9. When the pump’s linewidth becomes larger, the converted signal increased by factor 10.