Measuring device for polarization-maintaining optical fiber spindle differential delay

Abstract

A measurement device for polarization-maintaining optical fiber spindle difference delay is provided. The measurement device comprises a polarization-maintaining fiber (PM) Sagnac interferometer, a signal generator, a microwave detector, a microprocessor. The PM Sagnac interferometer comprises a laser, a photoelectric modulator, and a PM fiber coupler that are connected in sequence. The PM Sagnac interferometer further comprises an optical fiber interface J1 and an optical fiber interface J2 arranged at the two output ends of the PM fiber coupler, a PM fiber to be measured located between the fiber interface J1 and the fiber interface J2, and a photodetector arranged at the other output end of the PM fiber coupler.

Claims

1. A measurement device for polarization-maintaining (PM) optical fiber spindle differential delay, comprising: a PM fiber Sagnac interferometer; a signal generator; a microwave detector; and a microprocessor; wherein the PM fiber Sagnac interferometer comprises a laser, a photoelectric modulator and a PM fiber coupler, which are connected in sequence; the PM fiber Sagnac interferometer further comprises a first fiber interface and a second fiber interface arranged respectively at a first output port and a second output port of the PM fiber coupler, a to-be-measured PM fiber located between the first fiber interface and the second fiber interface, and a photoelectric detector arranged at a third output port of the PM fiber coupler; a low-coherence linear polarized light emitted by the laser is applied with a RF (radio frequency) signal by the photoelectric modulator, and divided into two output lights; a first output light of the two output lights along a fast axis of the PM fiber coupler enters the to-be-measured PM fiber through the first fiber interface and a first end of the to-be-measured PM fiber, and transmits along the fast axis of the to-be-measured PM fiber; a second output light of the two output lights along the fast axis of the PM fiber coupler is coupled to a slow axis of the to-be-measured PM fiber through the second fiber interface and a second end of the to-be-measured PM fiber; the two output lights are transmitted in opposite directions and in different axes; the first output light in the fast axis of the to-be-measured PM fiber is redirected into the slow axis of the to-be-measured PM fiber through the second fiber interface when the first output light arrives at the second end of the to-be-measured PM fiber; the second output light in the slow axis of the to-be-measured PM fiber will not change transmission axis when entering the slow axis of the PM fiber coupler; the microwave detector is arranged at an output end of the photoelectric detector and is configured to detect the power of the RF signal output by the photoelectric detector; the microprocessor is connected to the microwave detector, and is configured to calculate a light delay generated by the to-be-measured PM fiber according to an output signal of the microwave detector.

2. The measurement device according to claim 1, wherein the laser is a broad-spectrum light source with a coherence length less than 30 μm; the coherence length of the laser is much smaller than the optical path difference caused by different propagation constants of the fast and slow axes of the to-be-measured PM fiber.

3. The measurement device according to claim 2, wherein the laser emits linearly polarized light.

4. The measurement device according to claim 1, wherein the PM fiber coupler is a 2×2, 3 dB PM fiber coupler.

5. The measurement device according to claim 2, wherein two fibers on the PM fiber coupler that connect to the first fiber interface and second fiber interface have identical length.

6. The measurement device according to claim 1, wherein the optical fiber spindle differential delay is calculated by the microprocessor based on the following formula: $\tau =\frac{1}{.Math.f_{RF0}-f_{RF1}.Math.}$ wherein, f.sub.RF0 and f.sub.RF1 are the frequency values at which the voltage output by the adjacent microwave detector is 0.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to more clearly describe the technical solutions in the embodiments of the present disclosure, the following will briefly introduce the drawings that need to be used in the description of the embodiments. Obviously, the drawings in the following description are only exemplary embodiments of the present disclosure. For those of ordinary skill in the art, other drawings may be obtained based on these drawings without creative work.

(2) FIG. 1 is a schematic diagram of the measurement device for polarization-maintaining optical fiber spindle differential delay.

(3) FIG. 2 is a schematic diagram of the PM fiber.

(4) FIG. 3A-3B are schematic diagrams of the fiber interface, wherein, FIG. 3A shows the connection mode for the fiber interface J2; FIG. 3B shows the connection mode for the fiber interface J1.

DETAILED DESCRIPTION OF THE DISCLOSURE

(5) In order to describe the present disclosure more specifically, the technical solution of the present disclosure will be described in detail below with reference to the drawings and specific embodiments.

(6) As shown in FIG. 1, a measurement device for polarization-maintaining optical fiber spindle difference delay is provided. The measurement device comprises a laser (DFB) 1, a signal generator (Signal Source) 2, a photoelectric modulator (MZM) 3, a PM fiber coupler (OC) 4, a fiber interface J1, a fiber interface J2, a to-be-measured PM fiber 5, a photoelectric detector (PD) 6, a microwave detector (Radio Detector) 7 and a microcontroller (MCU) 8.

(7) A Sagnac interferometer is formed by the laser 1, photoelectric modulator 3, the PM fiber coupler 4, the fiber interface J1, the fiber interface J2, the photoelectric detector 6 and the to-be-measured PM fiber 5. As shown in FIG. 2, the to-be-measured PM fiber 5 comprises two transmission axis, one is the fast axis, the other is the slow axis which is perpendicular to the fast axis. As shown in FIG. 3A and FIG. 3B, the fiber interface J1 and fiber interface J2 are configured to connect the PM fiber coupler 4 and the to-be-measured PM fiber 5. Via the fiber interface J1, the fast axis of the PM fiber coupler 4 is aligned with fast axis of the to-be-measured PM fiber 5, and the slow axis of the PM fiber coupler 4 is aligned with the slow axis of the to-be-measured fiber 5. Via the fiber interface J2, the fast axis of the PM fiber coupler 4 is aligned with the slow axis of the to-be-measured fiber 5, and the slow axis of the PM fiber coupler 4 is aligned with the fast axis of the to-be-measured fiber 5.

(8) More specifically, the laser 1 is a broad-spectrum light source with a coherence length less than 30 μm. The coherence length of the laser 1 is much smaller than the optical path difference caused by the different propagation constants of the fast and slow axes of the PM fiber. Furthermore, the laser 1 emits linearly polarized light.

(9) More specifically, the PM fiber coupler 4 is a 2×2, 3 dB PM fiber coupler, and the two optical fibers on the PM fiber coupler 4 that connecting to the fiber interface J1 and fiber interface J2 have identical length.

(10) During operation, after the low-coherence linearly polarized light emitted by the laser 1 is carried with a RF signal via the photoelectric modulator 3, the PM fiber coupler divides the low-coherence linearly polarized light into two output lights. A first output light of the two output lights at port b of the PM fiber coupler 4 along the fast axis of the PM fiber coupler 4 enters the to-be-measured PM fiber through the optical fiber interface J1 and transmits along the fast axis of the to-be-measured PM fiber 4. A second output light of the two output lights at port c of the PM fiber coupler 4 along the fast axis of the PM fiber coupler 4 is coupled to the slow axis of the to-be-measured PM fiber 5. The two output lights are transmitted in opposite direction and in different axis. The first output light in the fast axis of the to-be-measured PM fiber 5 is redirected into the slow axis of the to-be-measured PM fiber 5 through the optical interface J2 when the first output light arrives at another end of the to-be-measured PM fiber 5. The second output light in the slow axis of the to-be-measured PM fiber 5 will not change its transmission axis when it enters the slow axis of the PM fiber coupler 4. At last, the two output lights are overlaid and output through the port d of the PM fiber coupler 4, and the optical signal of the overlaid light is converted into electrical signal by the photodetector 6.

(11) The frequency sweeping signals generated by the signal generator 2 are modulated onto the light and enter the PM fiber Sagnac interferometer. Since the propagation constants of the fast axis and the slow axis of the to-be-measured PM fiber are different, there is a time delay difference between the light on the two axes when the lights on the two axes arrive at the output end. The overlaid light signal is therefore converted into electrical signal. The amplitude of the converted signal carries the time delay information. Accordingly, the optical time delay generated by the to-be-measured PM fiber 6 can be calculated by the microwave detector 7 and the microcontroller 8.

(12) The work principle of the measurement device for the PM optical fiber spindle differential delay is as follows.

(13) The RF signal generated by the signal generator 2 may be expressed as:
V.sub.RF(t)=V.sub.RF cos(ω.sub.RFt)  (1)

(14) Where, V.sub.RF represents the amplitude of the RF signal; ω.sub.RF represents the frequency of the RF signal.

(15) The bias voltage applied to the photoelectric modulator is:
V.sub.in(t)=V.sub.DC+V.sub.RF cos(ω.sub.RFt)  (2)

(16) The resulting light phase change is:
φ.sub.bias(t)=πV.sub.DC/V.sub.π/V.sub.RF cos(2πf.sub.RFt)/V.sub.π  (3)

(17) Where, V.sub.DC is the DC (direct voltage) provided by the regulated DC power supply, and V.sub.π is the half-wave voltage of the photoelectric modulator. The first part of the formula (3) is the phase change produced by the DC bias, and the second part is the phase change produced by the modulation signal. When the initial phase is

(18) $\frac{\pi }{2},$
and the input signal is a small signal, the changes of the laser power tend to be liner. Therefore, under normal circumstances, the bias point of the photoelectric modulator must be placed at the half-wave voltage, that is,

(19) $V_{DC}=\frac{V_{\pi }}{2},$
so that the first-order electrical signal gain used in the product can be maximum value, and at the same time can well suppress high-order harmonic signals.

(20) The laser light modulated by the microwave can be expressed at the output end of the photoelectric modulator as follows:

(21) $\begin{array}{cc}{P}_{out}\left(t\right)=\frac{1}{2}{\alpha }_{\mathrm{loss}}{P}_0\left[1+\cos \left(\frac{\pi V_{DC}}{V_{\pi }}+\frac{\pi V_{RF}\cos \left(2\pi f_{RF}t\right)}{V_{\pi }}\right)\right]& \left(4\right)\end{array}$

(22) Where, α.sub.loss is the loss of the photoelectric modulator; P.sub.0 is the light intensity input by the laser; P.sub.out(t) is the light intensity output by the photoelectric modulator. The light output by the photoelectric modulator is divided into two output lights, by a 2×2, 3 dB PM fiber coupler, to transmit along different axes of the PM fiber. A first output light of the two output lights is transmitted along the fast axis of the PM fiber, while a second output light of the two output lights is coupled to the slow axis of the to-be-measured PM fiber via a polarization controller. The light signals in the fast and slow axes of the PM fiber can be expressed as follows.

(23) $\begin{array}{cc}{P}_{fast}=\frac{1}{4}{\alpha }_{\mathrm{loss}}{P}_0\left[1+\cos \left(\frac{\pi V_{DC}}{V_{\pi }}+\frac{\pi V_{RF}\cos \left(2\pi f_{RF}t+{\mathrm{\Delta \phi }}_f\right)}{V_{\pi }}\right)\right]{\mathrm{\Delta \phi }}_f=2\pi f_{RF}{\tau }_f& \left(5\right)\\ P_{slow}=\frac{1}{4}{\alpha }_{\mathrm{loss}}{P}_0\left[1+\cos \left(\frac{\pi V_{DC}}{V_{\pi }}+\frac{\pi V_{RF}\cos \left(2\pi f_{RF}t+{\mathrm{\Delta \phi }}_s\right)}{V_{\pi }}\right)\right]{\mathrm{\Delta \phi }}_s=2\pi f_{RF}{\tau }_s& (60\end{array}$

(24) Where, Δφ.sub.f is the amount of phase change produced by the RF signal in the fast axis; Δφ.sub.s is the amount of phase change produced by the RF signal in the slow axis; τ.sub.f is the time delay of the RF signal in the fast axis; τ.sub.s is the time delay of the RF signal in the slow axis. Since the laser is a low-coherence source, the following condition is meet:

(25) $\begin{array}{cc}\tau .Math.f_{RF}.Math.L>\frac{{\lambda }_0^2}{\Delta \lambda }& \left(7\right)\end{array}$

(26) Where, f.sub.RF is the frequency of the RF signal; L is the beat length of the to-be-measured PM fiber; τ is the time delay difference generated when light propagates on the fast axis and the slow axis of the to-be-measured PM fiber. Therefore, the two output lights transmitting along the fast axis and the slow axis are overlaid and output by the PM fiber coupler, and the output optical signal is sent to the photodetector. Ignoring the DC component, the output current of the first-order signal can be obtained as:

(27) $\begin{array}{cc}\begin{array}{c}{I}_f=\eta {\alpha }_{1oss}{P}_0{J}_1\left(\frac{\pi V_{\pi }}{V_{\pi }}\right)\left[\cos \left(2\pi f_{RF}t+{\mathrm{\Delta \phi }}_s\right)+\cos \left(2\pi f_{RF}t+{\mathrm{\Delta \phi }}_f\right)\right]\\ =2\eta {\alpha }_{\mathrm{loss}}{P}_0{J}_1\left(\frac{\pi V_{\pi }}{V_{\pi }}\right)\cos \left(\frac{{\mathrm{\Delta \phi }}_s-{\mathrm{\Delta \phi }}_f}{2}\right)\\ \cos \left(2\pi f_{RF}t+\frac{{\mathrm{\Delta \phi }}_s+{\mathrm{\Delta \phi }}_f}{2}\right)\end{array}& \left(8\right)\end{array}$

(28) Where, η is the photoelectric conversion efficiency and α.sub.loss is the loss of the photoelectric modulator. The output signal of the photodetector is amplified and then input to the microwave detector to obtain the output signal:
V.sub.out=|cos[2πf.sub.RF(τ.sub.s−τ.sub.f)]|  (9)

(29) As what is apparent, the output voltage V.sub.out and the frequency f.sub.RF satisfy the cosine relationship. The period of the cosine function is related to the value of (τ.sub.s−τ.sub.f). As long as the two adjacent frequency points f.sub.RF0 and f.sub.RF0 are measured, the delay amount can be obtained:

(30) $\begin{array}{cc}\tau ={\tau }_s-{\tau }_f=\frac{1}{.Math.f_{RF0}-f_{RF1}.Math.}& \left(10\right)\end{array}$

(31) The provided device uses different propagation constants of light in the fast axis and slow axis of the PM fiber to generate a delay difference to measure the length of the to-be-measured fiber. The structure of the Sagnac interferometer is adopted, and the reference optical path is not need. The influence of temperature on measurement accuracy is reduced. At the same time, combined with the time delay measurement method based on microwave photon technology, the measurement accuracy of the provided device can reach the picosecond level and the dynamic range of which can be the kilometer level.

(32) The above description of the embodiments is to facilitate those of ordinary skill in the art to understand and apply the present disclosure. It is obvious that those skilled in the art can easily make various modifications to the above-mentioned embodiments, and apply the general principles described here to other embodiments without creative efforts. Therefore, the present disclosure is not limited to the above-mentioned embodiments. According to the disclosure of the present disclosure, the improvements and modifications made to the present disclosure by those skilled in the art are within the scope of the present disclosure.