Improvement of high sensitivity ICR characteristics and its application in OFDR

Coherent optical sensing technology faces significant challenges in practical applications due to the extremely weak signal light, necessitating highly sensitive coherent receivers. A high-sensitivity, highly integrated, and low-noise coherent receiver module based on germanium-silicon photonic chips has been developed. A testing system was constructed to assess the performance of the coherent receiver module, revealing a reception sensitivity of -70dBm, with physical dimensions of 33×40×8.6 mm3 and an output noise voltage of ±4mV. Additionally, a distributed sensing system based on optical frequency domain reflectometry (OFDR) was established to verify the module’s application in OFDR systems, achieving a spatial resolution of 1mm.


Introduction
The coherent receivers available in the market are mostly modular devices composed of discrete components such as optical mixers, PD arrays, and gain amplification circuit boards [1].They have low integration and large physical dimensions, which no longer meet the requirements for future coherent receiver applications.For coherent optical sensing technology and applications, high sensitivity and high integration mean better transmission capability and more application scenarios [2].High-sensitivity ICR (Integrated Coherent Receiver) is one of the key technologies in coherent communication and coherent optical sensing [3].In this paper, to address the issues of high sensitivity, high integration, and low noise in coherent receivers, we propose the integration of optical mixers and photodiodes using germanium-silicon photonic chips to reduce hardware size [4].Furthermore, balanced detection technology and low-noise amplification circuits are used to improve sensitivity, enhance anti-interference capabilities, and effectively suppress relative intensity noise and quantum noise in the local oscillator light.Theoretical analysis is conducted on the impact of the integrated coherent receiver scheme utilizing Silicon photonic chip, balanced detectors, and gain amplification circuits on the receiver sensitivity and integration level in coherent optical sensing technology.At the same time, tests were conducted to evaluate the receiver sensitivity and noise immunity of the module when operating within a bandwidth of 100 MHz.Within a 100 MHz bandwidth, the integrated coherent receiver revealing a reception sensitivity of -70dBm, with physical dimensions of 33×40×8.6 mm³ , and an output noise voltage of ±4 mV.

Device Design
The coherent receiver consists of an optical mixer, a PD array, and a gain amplification circuit.In this module, a germanium-silicon chip is integrated with the optical mixer and PD array.From an external perspective, the coherent detection module consists of a germanium-silicon chip and a gain amplification circuit (as shown in Figure 1).The gain amplification circuit board has a groove, and the germanium-silicon chip is bonded to the groove of the gain amplification circuit board using silver adhesive.The germanium-silicon chip is connected to the gain amplification circuit board through wire bonding, and the gain amplification circuit board is connected to the external circuit, achieving electrical interconnection between the chip and the external circuit.The germanium-silicon chip adopts FA optical coupling to establish a connection between the chip and the optical signal.

Germanium-Silicon Optical Chip
The waveguide material used in the chip of this scheme is silicon, and the detector material is germanium, which integrates optical mixer and PD array.Common ICRs use 90-degree mixers in coherent optical communication systems [5].The ICR in this paper is mainly used in optical fiber sensing systems, and its signal light is not modulated.Therefore, a simple 180-degree mixer can be used at the receiving end, which can simplify the components, reduce costs, and shrink the size.The PD array adopts balanced detection technology [6].The key components of this silicon photonic chip include a polarization splitter rotator (PSR), a splitter, two couplers (Hybrid), and four PIN-type photodetectors.The signal light is coupled into the chip and first split by the PSR into TE and TM modes, where the TM mode needs to be rotated into the TE mode.The output ends of the PSR are connected to two Hybrids, respectively.The local oscillator light is also coupled into the chip, divided into two outgoing beams by the Hybrid, and enters two Hybrids, respectively.The two Hybrids perform coherent mixing, and finally the optical signals are converted into electrical signals by two sets of balanced detectors, namely four photodetectors.
After being optically coupled by the FA, the germanium-silicon chip (as shown in Figure 2) receives local light and signal light respectively, and the optical signal enters the germanium-silicon photonic chip for optical mixing.The local oscillator light is evenly divided into two beams, which are coherent with the TE and TM modes of the signal light, respectively.The TM mode is rotated and converted into the TE mode, and then the balanced detection technology is used.The output optical signal is detected and received by two high-speed and low-noise balanced detectors, obtaining the polarization coherent signal.Performance testing and analysis show that the main parameters of the germanium-silicon photonic chip are: PD responsivity: 0.8A/W@1550nm, dark current <50nA, operating voltage -3V, covering C-band.End-face coupler: single-end insertion loss ≤ 2.5dB, operating in the C-band.

Balanced detection technique
The working principle of balanced detection technique is illustrated in  The optical signal from the output of the coupler is converted into an electrical signal by the photodiode.The generated photocurrent is proportional to the power of the optical signal, and the optical power is proportional to the square of the electric field strength of the optical signal.Assuming a coupling ratio of 50:50 for the coupler and equal responsivities for both detectors, the photocurrent difference Δi can be expressed as follows: In the above equation,   L S    represents the obtained intermediate-frequency signal.From the expression of the output current of the balanced detector, it can be observed that the output reflects the amplitude, frequency, and phase information of the signal light.Therefore, this coherent receiver can be applied not only in conventional OTDR technology but also in OFDR technology, which combines optical frequency domain analysis with optical heterodyne detection.A linearly swept laser emits light and distributes it to the signal arm and the reference arm.The light reflected back from each position in the signal arm fiber interferes with the reference light, forming beat frequency interference.The received signal frequency and intensity are utilized to determine the location and characteristics of events.

Gain amplification circuit
After obtaining the polarization coherent signal in the germanium-silicon photonic chip, the signal is transmitted to the gain amplification circuit through gold wire.The main function of the gain amplification circuit includes: converting the current signal into a voltage signal via the transimpedance amplifier, amplifying the weak signal, and filtering the signal through the filter circuit.As shown in Figure 4  According to the requirements, the TIA gain R1 is set to 6.25KΩ and the bandwidth fP is 100MHz.Therefore, the feedback capacitor C1 and the gain-bandwidth GBW need to be satisfied: As shown in Figure 5, R1=6.25kΩ and C1=0.1pF were selected.Because the OPA855 has a low closed-loop output impedance, a 50 ohm resistor is connected in series at the output of the operational amplifier to balance the impedance between the operational amplifier output and the impedance of the subsequent receiver.

Low-pass filter.
After the voltage signal is output from the transimpedance amplifier circuit, it flows into the low-pass filter circuit.In order to prevent the DC component in the signal from causing nonlinearity or saturation of the operational amplifier output, and to limit the level of output noise, a low-pass filter with a passband of DC to 100MHz is used for filtering.A simple π-type filter structure is used, and the filter order is set to 3rd order to accelerate the out-of-band attenuation, with the center frequency set to 50MHz.As shown in the figure below.

OPA.
As the TIA amplifies both the signal and noise, gain and noise matching need to be considered when designing the TIA amplification stage.In general, low noise is prioritized, and if the output amplitude has not reached the optimal sampling range of the acquisition circuit, further signal amplification can be performed to improve the detection system performance.Figure 7 shows a voltage amplification circuit, where R7 is connected in parallel to match the output impedance of the previous stage because the input impedance of the operational amplifier's inverting input is in the kiloohm range.The amplification factor of the circuit is determined by the resistance values of R4 and R5, and the signal is amplified by 6.5 times in this case.
Filtering of the power supply in the circuit: Capacitors have an equivalent circuit that contains resistors and inductors, and each capacitance value has a resonant point with the best filtering effect at the resonant frequency.After the frequency exceeds the resonant point, the capacitor exhibits inductive characteristics.The use of a combination of two capacitors with a 100-fold difference in capacitance value has better characteristics when stacked.In this article, 0.1uF and 10uF capacitors were used for power supply filtering.

Performance testing
The ICR testing system, as shown in Figure 8, consists of a 1550nm light source that emits light waves.The light is split 1:1 by a coupler, with one path serving as the local oscillator light and entering the coherent receiver.The other path enters a modulator, where it is modulated by an RF signal source.It then passes through a variable attenuator for signal attenuation before being directed into the coherent receiver as the signal light.The two demodulated signals from the coherent receiver are connected to an oscilloscope for testing.

Output noise voltage
During the testing of the gain amplification circuit's noise, with no incident light, only power is supplied to the coherent receiver.The noise level is determined by observing the voltage on an oscilloscope.Currently, leading discrete component coherent receivers on the market have the output noise voltage controlled within ±5mV.Testing has demonstrated that the output noise voltage of this integrated coherent receiver is within ±4mV.

Sensitivity testing
By using a variable attenuator, the optical power of the signal light is adjusted to -70dBm from an initial power level of 3dBm.At this setting, a pulsed signal is generated by an RF signal source at 5MHz, and after detection and reception by the integrated coherent receiver, the input oscilloscope displays the results as shown in Figure 9.Under the aforementioned conditions, the coherent receiver detects a signal amplitude of approximately 30dBm.

Application in OFDR distributed sensing system
The optical path structure of the OFDR sensing system is illustrated in Figure 10, which mainly includes a light source, an optical coupler, an optical circulator, a polarization beam splitter, an integrated coherent receiver (ICR), a data acquisition card, a computer, and single-mode optical fibers [7].The splitting ratio of the optical coupler OC1 is 50:50.Half of the swept-frequency laser output from the light source enters the main interferometer's optical coupler OC2, while the remaining 50% goes into the auxiliary interferometer's optical coupler OC4.The splitting ratio of the optical coupler OC2 is 99:1, with 99% of the incident light entering the main interferometer's test fiber and then the optical circulator's port 1.The remaining 1% of the main interferometer's incident light enters the developed coherent receiver module (ICR).Port 2 of the optical circulator is connected to the test fiber, and the Rayleigh backscattered light from the test fiber enters port 3 of the optical circulator and is combined with the reference light to enter the ICR.The splitting ratio of the optical coupler OC3 is also 50:50, with 50% of the light directly entering the ICR and the other 50% passing through a delay fiber before entering the ICR.Similarly, the main interferometer and auxiliary interferometer's beat frequency interference electrical signals are received by the ICR and converted into electrical signals.The beat frequency interference signals from the main interferometer and auxiliary interferometer are transmitted to the data acquisition card, which samples the beat frequency signals using the TSL-770 Trigger signal as a trigger.Finally, the discrete data obtained from sampling is transmitted to the computer for algorithm processing and to obtain the final distance-domain result [8].During offline data processing in MATLAB software, a one-dimensional interpolation algorithm is used to compensate for the non-linear effects of the light source.As shown in Figure 11, the breakpoints at 2.13m and 5.18m are the flange positions at the two ends of the optical circulator in the main interferometer, and the breakpoint at 6.25m is the position of the end of the fiber under test (FUT).Amplifying the breakpoint at 5.18m, as shown in Figure 12, a spatial resolution of 0.6mm is measured.

Conclusions
After testing, the integrated coherent receiver module developed in this paper achieved a reception sensitivity of -70dBm for unmodulated signal light without requiring demodulation at the ICR receiving end.The physical dimensions of the module were reduced to 33×40×8.6 mm³ and its output noise voltage was decreased to ±4mV.Furthermore, the module was successfully applied to a distributed sensing system based on optical frequency domain reflectometry (OFDR), verifying its application in OFDR systems and achieving a spatial resolution of 1mm.In reference to the received optical power, a pulse laser phase-locking and zero-crossing detection method was proposed by the Optical Communication Team of the University of Electronic Science and Technology of China in 2020 [9].Experimental results demonstrated pulse laser phase-locking from 50 kHz to 2.4 MHz, with zero-crossing detection still achievable even at peak powers as low as -75 dBm.Compared with the international leading level, the ICR developed in this paper was applied to optical fiber sensing systems and requires only signal detection without demodulation, resulting in lower difficulty compared to coherent optical communication.Thus, only a rough comparison can be made between the two types of ICRs, but currently, the reception sensitivity of the ICR developed in this paper (-70dBm) has reached a high level.

Figure 3 .
ES and EL denote the signal light and local oscillator light, respectively.They undergo optical coupling to generate corresponding reflected light and transmitted light, denoted as ESR,EST,ELR,ELT.The reflected light ESR of the signal light and the transmitted light ELT of the local oscillator light are incident on the photodiode PIN1, generating photocurrent i1.The transmitted light EST of the signal light and the reflected light ELR of the local oscillator light enter the photodiode PIN2, generating photocurrent i2.

Figure 3 .
Figure 3. Balanced detection technique.The optical signal from the output of the coupler is converted into an electrical signal by the photodiode.The generated photocurrent is proportional to the power of the optical signal, and the optical power is proportional to the square of the electric field strength of the optical signal.Assuming a coupling ratio of 50:50 for the coupler and equal responsivities for both detectors, the photocurrent difference Δi can be expressed as follows: , In the silicon photonic chip, the optical signals emitted by two 180-degree mixers are detected by two sets of balanced detectors, thus two optical currents enter the gain amplification circuit.The two optical currents go through the transimpedance amplifier, filter circuit, and second-stage amplifier, respectively, and the two gain amplification circuits output RF signals.Finally, the two signals are added and analyzed and processed in instruments such as oscilloscopes.Both signals are converted from current to voltage through the first-stage transimpedance circuit, and after passing through the low-pass filter circuit, the total gain is increased in the second-stage amplification circuit.In the following, the designed TIA, Filter, and OPA are introduced.

Figure 4 .
Figure 4. ICR.2.3.1.TIA.For this design, the OPA855 was selected as the TIA, with a gain-bandwidth product of 8GHz, input voltage noise, input capacitance of 0.6pF (common mode) and 0.2pF (differential), wide input common mode range, and a static current of 17.8mA.It meets requirements for low noise, high open-loop gain, low input bias current, high bandwidth, and low output impedance.According to the requirements, the TIA gain R1 is set to 6.25KΩ and the bandwidth fP is 100MHz.Therefore, the feedback capacitor C1 and the gain-bandwidth GBW need to be satisfied: