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Broadband RF signals can be efficiently transmitted over frequency modulated (FM) RF photonic links (Fig. 1), which offer advantages over traditional microwave links. The advantages include large transmission bandwidth, immunity to electromagnetic interference, low attenuation, and high link efficiency. In systems employing a direct detection scheme, FM signals need to be converted to intensity modulated (IM) signals by frequency discriminators. A frequency discriminator is usually an optical filter whose transmission depends on the input light frequency. The linearity of the frequency discriminator is crucial to the dynamic range of an analog RF photonic link.
Fig. 1. Analog RF Photonic Link
The setups of Sagnac filter discriminator and our proposed modified ring-assisted Sagnac filter discriminator is shown below(Fig. 2). The PM fibers used are 1.25 meters long and have birefringence of 4.4x10 -4 .
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| (a) |
(b) |
Fig. 2 (a) Standard Sagnac filter (b) Ring-assisted Sagnac filter frequency discriminator
We have obtained the simulated and measured transfer function spectra with a broadband source and an OSA for both devices.
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| (a) |
(b) |
Fig. 3 Transmission spectrum of (a) simple Sagnac filter (measured) (b) Ring-assisted Sagnac filter (measured vs. modeled)
To compare the performances of the two devices, we normalize the measured spectra and fit them with third-order polynomials around the quadrature frequency. The results are shown in Fig. 4.
Fig. 4 Measured frequency responses of a standard Sagnac FD and the proposed ring-assisted Sagnac FD around the quadrature frequency, with an ideal straight line as the reference
Table I. Comparison of modeled and measured transfer function coefficients, the corresponding maximum modulation strength Δν, and output power range for which the intermodulation distortion is at least 25 dB below the fundamental signal level of a standard Sagnac frequency discriminator and the proposed ring-assisted Sagnac frequency discriminator
| Δ PAM ≈ ½ PFM ( AΔν + BΔν2 + CΔν3 ) |
A
( GHz -1 ) |
B
( GHz -2 ) |
C
( GHz -3 ) |
Δν
( GHz ) |
Δ PAM |
Standard Sagnac FM
discriminator |
theory |
1.17E-2 |
0 |
-2.67E-7 |
± 11.8 |
± 0.0687PFM |
| experiment |
1.17E-2 |
2.93E-6 |
-5.35E-7 |
± 8.3 |
± 0.0486PFM |
Ring assisted Sagnac FM
discriminator |
theory |
1.52E-2 |
5.04E-5 |
-2.00E-7 |
± 15.5 |
± 0.120PFM |
| experiment |
1.45E-2 |
1.52e-5 |
-2.37E-7 |
± 14.0 |
± 0.102PFM |
As can be seen from the above table, in order to have intermodulation suppression to be larger than 25dB, we need to limit the IM power swing to be ±0.0486PFM or about 10% for standard Sagnac frequency discriminator. However for the proposed device, this results in the IM power swing of ±0.102PFM or over 20%. In terms of the dynamic range of an optical filter, the ring-assisted Sagnac frequency discriminator offers more than 3dB improvement over the standard Sagnac discriminator. This corresponds to a factor of 2 increase in voltage level of the detected RF signals, which leads to a 6 dB improvement in the third-order input intercept point (IP3). Assuming that the noise level remains the same for the RF link, the proposed discriminator offers a 4dB improvement in the dynamic range of the receiver.
Reference:
Gang Chen, Jin U. Kang and Jacob B Khurgin, “Frequency Discriminator based on
Ring-Assisted Fiber Sagnac Filter,” IEEE Photonic Technology Letters, accepted for publication |
