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A research group from the University of Alabama in Huntsville has developed a simple, compact, and robust technique for demodulating PM/FM optical signals. They use the reflection characteristics of fiber Bragg gratings written in polarization-maintaining fibers to create a frequency discriminator, which is able to convert PM/FM signals into intensity-modulated (IM) signals.

The conventional techniques for demodulating phase/frequency-modulated (PM/FM) optical signals either require a sophisticated frequency-stable laser or employ a discriminator with poor characteristics. This efficient technique features high linearity over a wide bandwidth along with low background noise and can potentially find applications in optical communications, microwave photonics, and radar/lidar technologies.

"The new technique combines the advantages of FBGs and birefringent crystals by employing an FBG fabricated in a polarization-maintaining (PM) fiber, i.e., a PM-FBG. Such a combination allows the demodulator to enjoy the low cost and compactness of an FBG while benefitting from the rigidity and the common optical path of a birefringent material", said Dr. Dipen Barot, a postdoctoral researcher from NIST/CU.

Because of the birefringence property of the PM fiber, a single FBG results in two identical reflection peaks with a slight wavelength offset along the two orthogonal polarizations, alleviating the burden of fabricating two identical FBGs. The double-peak profile also provides a natural quadrature point with equal and opposite slopes. This allows a PM-FBG to perform background-free linear operations.

A proper PM-FBG is chosen so that its two resonance peaks cross near their 3dB points (for maximum slope). An ideal optical oscillator with a single frequency is first tuned to the crossover wavelength between the two Bragg reflection peaks. The polarization of the injected optical carrier is adjusted so that an equal amount of optical power is reflected by the PM-FBG along its fast and slow axes. The two orthogonally polarized reflection signals are separately detected, and the detector outputs are subtracted from each other to create a null (i.e., balanced photodetection (BPD)). This effectively creates a quadrature point. 

"When the frequency of the optical carrier is dislocated from the crossover wavelength, the reflected powers along the fast and the slow axes change toward opposite directions due to the opposite signs of their corresponding reflectivity slopes, said Rui Zhou, a researcher from the University of Alabama in Huntsville. This allows the BPD to generate a large response. Moreover, when the frequency of the optical carrier is modulated near the crossover wavelength, the reflected optical intensities along the fast and the slow axes also vary periodically but with a 180-degree phase difference. This leads to a large, background-free response at the output of the BPD. In essence, the above scheme allows the PM-FBG to serve as an optical frequency discriminator, which in turn performs FM demodulation."

The PM-FBG is fabricated on a PANDA-type PM fiber. It has two Bragg reflection peaks about 0.6 nm apart, with each peak having a 0.4-nm full width at half maximum (FWHM). A tunable external-cavity diode laser operating near 1550 nm serves as the light source. Its wavelength is tuned to the crossover wavelength of the two Bragg peaks. The laser output passes through an isolator before entering a polarization controller (PC). The polarization controller is used to set the polarization state of the light. A polarization-maintaining 50:50 coupler takes the output of the PC, feeds it into the PM-FBG, and directs the reflected power toward the output. A fiber-coupled polarization beam splitter splits the two orthogonal polarization modes in the output, feeding them into the two photodiodes of a balanced photoreceiver, which provides a trans-impedance gain of 5.1×104 V/A. The input of the demodulator is PM/FM-modulated via an electro-optic modulator (EOM), which is driven by an RF driver. All the fibers and fiber connectors after the polarization controller are PM type so that the polarization state is preserved.

To find out the transfer characteristics of the PM-FBG demodulator, the wavelength of the tunable diode laser is swept within the reflection spectrum of the PM-FBG, and the reflected light in the two orthogonal polarization states is separately detected. Additional tests of linearity are performed by measuring the demodulated signal level versus the FM modulation index. An increase in the FM modulation index results in an increase in the carrier displacement from the quadrature point. The background noise in the output of an FM demodulator mainly comes from carrier leakage and the unwanted response to IM. To measure the IM response of our FM demodulator, they purposely applied IM along with FM by placing an intensity modulator in series with the EOM. A small fraction (10%) of the intensity-modulated light is measured with a separate photodiode, and the rest of the light is applied to the FM demodulator. The recovery of frequency-modulating signals with high fidelity is another crucial aspect of FM demodulators. To examine the fidelity of the PM-FBG demodulator, an arbitrary periodic signal at a frequency of 5 MHz is applied to the EOM, and the recovered FM signal is compared with the original driving signal. 

Long-term stability is a critical metric for FM demodulators. In the case of PM-FBG, temperature variations can lead to Bragg wavelength shifts and polarization state fluctuations. Both of these effects cause the BPD output to drift away from the null without carrier modulation. Such drift may increase background noise and introduce distortion in the output. To verify the tolerance of the demodulator against temperature fluctuations, they monitored the BPD output without any input FM signals in a normal lab environment for long periods of time. It is found that, even with a random drift over time, the BPD output remains close to zero within about 50 mV for extended periods of time (sometimes hours). This is in comparison with the maximum output range from the BPD. The result shows that the PM-FBG demodulator can maintain good stability under a reasonably controlled environment. Dr. Lingze Duan said "This method features high linearity, large bandwidth, and background-free operation. We hope that this work opens up a new avenue toward compact and low-cost optical PM/FM demodulators."

Click here to read the article titled, 'Optical phase/frequency demodulation using polarization-maintaining fiber Bragg Gratings.'