CN100547953C - The method of measurement of polarized dependent loss PDL - Google Patents

Abstract

A kind of method of measurement of polarized dependent loss PDL is characterized in that: make the incident modulated light signal enter a birefringence device, successively by a polarization rotator, a PDL device under test enters a polarimeter at last then; Make polarization rotator along polarization axle to rotating a circle, in this process, write down DOP maximum and the minimum value that is occurred respectively, and calculate corresponding devices PDL value, PDL=5log according to formula (13) ₁₀(1+DOP _Max-DOP _Min-DOP _MaxDOP _Min)/(1+DOP _Min-DOP _Max-DOP _MaxDOP _Min) advantage of (13) this method is: can effectively measure the PDL value in optical device under single wavelength or the multi-wavelength situation or the system fast; And can realize PDL intelligent measure under the no worker monitor situation, simple, FEEDBACK CONTROL is extremely simple, and has certain cost advantage.

Description

The method of measurement of polarized dependent loss PDL

Technical field

The present invention relates to a kind of method that realizes that Polarization Dependent Loss (PDL) is measured, the PDL that is used for fiber optic telecommunications equipment and system measures, and can realize the PDL feature measurement of sun adjuster spare and system under single wavelength or the multi-wavelength situation.

Technical background

PDL is the ratio of optical device or the system minimum and maximum transmitting optical power under all polarization states.In optical fiber telecommunications system, the PDL of optical device can cause the transmission signals distortion and finally cause the deterioration of system transmissions performance.Therefore, PDL is most important for the sign of optical device.In fact, each device all shows as a kind of polarization associated transport.Because the polarization of transmission signals not only is confined within the fiber optic network, so the insertion loss of device is different with polarization state.Because any variation of transmitting optical signal polarization state in the optical fiber, this effect can uncontrollably increase along transmission link, transmission quality is brought have a strong impact on.The PDL of individual devices can cause big power fluctuation in system, thereby has improved the bit error rate of system, even can cause network failure.In conjunction with polarization mode dispersion (PMD), PDL may become the main source of pulse distortion and diffusion.And in the wavelength selection type device of WDM network, PDL changes corresponding to the spectral transmissions indicatrix of device.In addition, some filter attribute (as fluctuation or pass band width) also is that polarization is relevant.Therefore, the mensuration of PDL wavelength characteristic is essential equally.Therefore, for many optic communication device manufacturers and user, one accurately and effectively the PDL measurement scheme just seem very important.

Existing P DL method of measurement can be divided into two big classes: Deterministic Methods and uncertainty method.Deterministic Methods is by measuring the transmission property of DUT under a series of definition input polarization states, from the Mueller of DUT or Jones test matrix, derive and draw its PDL, Mueller method for example, the i.e. scanning that execution has four kinds of different polarization states (at least one is ellipse, and common four kinds of polarizations are quadrature) for four times on selected wavelength interval.This method is that the PDL measurement is relevant with the every bit (or wavelength location) of scanning with the main distinction of disturbing between the folk prescription method.After four scanning, the user can select a measurement point every 0.05nm in 300nm.Yet, disturb inclined to one side method and do such work if use, inconceivable and be difficult to realize.The uncertainty method is measured minimum and the maximum transmitted optical power value of DUT under a large amount of input polarization states; For example disturb inclined to one side technology, promptly between light source and DUT, place scrambler, when covering Poincar é Sphere, measure loss and change, but part shows all possible polarization state like this.This method can produce point-device result, just can the tested person probe and polarization relevant response (PDR), the scrambler of internal reference probe activate loss (i.e. the variable power that under the situation that does not have PDL of system and probe PDR, measures) and covering the restriction of possible polarization state range size.The major defect of this technology is that it can not use with quick length scanning.

List of references

【1】Iannone，E.F.Matera，A.Meeozzi，and?M.Settembre，NonlinearOptical?Communication?Networks，New?York：John?Wiley?&?Sons，Inc.，pp.30-35，1998.

【2】Christopher?Allen，Pradeep?Kumar?Kondamuri，Douglas?L.Richards，Douglas?C.Hague.“Measured?Temporal?and?Spectral?PMDCharacteristics?and?Their?Implications?for?Network-Level?MitigationApproaches”.Journal?of?Lightwave?Technology，Volume?21，Issue?1，79-86，January?2003.

【3】Betti，S.F.Curti，B.Daino，G.De?Marchis，E.Iannone，and?F.Matera，“Evaluation?of?the?bandwidth?of?the?principle?states?of?polarization?insingle-mode?fibers”，Optics?Letters，16(7)，pp.467-469，1991.

Summary of the invention

The principle of regular variation can take place along with the PDL variation in the degree of polarization (DOP) of modulation signal during based on PDL and PMD coexistence, the invention provides a kind of method of measurement of polarized dependent loss PDL, the PDL that is used for fiber optic telecommunications equipment and system measures, and can realize the PDL feature measurement of sun adjuster spare under single wavelength or the multi-wavelength situation or system.

Technical scheme of the present invention is: a kind of method of measurement of polarized dependent loss PDL, it is characterized in that: make the incident modulated light signal enter a birefringence device, successively by a polarization rotator, a PDL device under test enters a polarimeter at last then; Make polarization rotator along polarization axle to rotating a circle, in this process, write down DOP maximum and the minimum value that is occurred respectively, and calculate corresponding devices PDL value according to following formula,

$\mathrm{PDL}=5{\log }_{10}\frac{1+{\mathrm{DOP}}_{\max }-{\mathrm{DOP}}_{\min }-{\mathrm{DOP}}_{\max }\·{\mathrm{DOP}}_{\min }}{1+{\mathrm{DOP}}_{\min }-{\mathrm{DOP}}_{\max }-{\mathrm{DOP}}_{\max }\·{\mathrm{DOP}}_{\min }}$

The method of measurement of aforesaid polarized dependent loss PDL is characterized in that: the light signal that enters the birefringence device is a linearly polarized light, the energy distribution equalization of light signal on birefringence device two polarization principal axis.

In our design, in order to simplify the measurement scheme of PDL, the light signal that enters the birefringence device is preferably linearly polarized light, and makes the energy distribution equalization of light signal on these birefringence device two polarization principal axis.For this reason, can adopt the combination of linear polarizer and birefringence device, promptly before the birefringence device, add a linear polarizer (both can be integrated in the practical application), and will adopt polarization maintaining optical fibre to be connected between light source and the linear polarizer.Along with polarization rotator rotates a week, polarimeter is measured the minimum and maximum value of corresponding light signal DOP, and feed back to control module, control module calculates the PDL value of PDL device under test, and according to the required PDL certainty of measurement that reaches, the corresponding computing of controlling the rotary state of polarization rotator at any time and improving the PDL certainty of measurement.And because generally a lot of DWDM device need be measured the PDL value under a plurality of wavelength situations, this can realize by the laser of selecting the wavelength-tunable type in Fig. 1.

Scheme preferably of the present invention is that the light modulated light signal that enters the birefringence device need be linearly polarized light, and the energy distribution equalization of light signal on these birefringence device two polarization principal axis.In order to simplify the measurement scheme of PDL, the light signal that enters the birefringence device is a linearly polarized light, and resulting DOP expression formula is the most succinct under the linearly polarized light situation, and then can increase the feasibility of this PDL measurement scheme.

Use the advantage of this method to be:

1. this method only needs the minimum and maximum value of measuring light signal DOP to obtain PDL value in optical device or the system, and unlike disturbing the folk prescription method, need to measure the polarization state of a large amount of random variations, thereby can effectively measure the PDL value in optical device under single wavelength or the multi-wavelength situation or the system fast; And can realize PDL intelligent measure under the no worker monitor situation.

2. measurement scheme is simple, and the FEEDBACK CONTROL scheme is extremely simple; And relatively has certain cost advantage with existing PDL measurement scheme.

Description of drawings

Fig. 1 is the PDL measurement scheme theory diagram of the embodiment of the invention.

Fig. 2 is an input polarization light signal in the embodiment of the invention, the fast/slow main shaft of birefringence device, and the relativeness schematic diagram between the PDL decay axle.

Fig. 3 has provided the flow chart of the feedback control algorithm of the embodiment of the invention.

Embodiment

For realizing purpose of the present invention, we provide method and the implement device thereof of realizing this purpose.Further specify the specific embodiment of the present invention and operation principle below in conjunction with accompanying drawing:

Fig. 1 is the PDL measurement scheme theory diagram based on the DOP measurement of the embodiment of the invention.

The method that as shown in fig. 1 realization multi-wavelength PDL measures is used to realize under single wavelength or the multi-wavelength situation PDL feature measurement to fiber optic telecommunications equipment and system.Mainly comprise in its scheme:

A modulated light signal generating means is used to produce the modulated light signal of single wavelength or multi-wavelength;

A linear polarizer is used for changing modulation signal light into linearly polarized light;

A birefringence device is used to produce Differential Group Delay (being single order PMD), and by regulating the relative angle between linear polarizer and birefringence device polarization principal axis, realizes light signal equipartition of energy on these birefringence device two polarization principal axis;

A polarization rotator is used for polarized light signal is rotated to needed polarization state from original state of polarization;

A feedback signal device is used for sensed light signal and offers control system by the state after the PDL device to be measured and with information;

The state of the information Control polarization rotator that provides according to feedback signal device is provided a control system.

Non-return-to-zero (NRZ) modulation signal (can be counted as a linear polarization signal) enters a birefringence device, and (this device is used to produce Differential Group Delay, be single order PMD), then successively by a polarization rotator, a PDL device under test (DUT) enters a polarimeter (this device is used to measure the DOP of input optical signal) at last.In the present embodiment, in order to simplify the measurement scheme of PDL, (resulting DOP expression formula is more succinct under the linearly polarized light situation light signal that enters the birefringence device can be adjusted to linearly polarized light, and then can increase the feasibility of this PDL measurement scheme), and make the energy distribution equalization of light signal on these birefringence device two polarization principal axis.For this reason, can before the birefringence device, add a linear polarizer (linear polarizer and birefringence device can be integrated in the practical application), and between light source and linear polarizer, adopt polarization maintaining optical fibre to be connected.Rotation along with polarization rotator, along with polarization rotator rotates a week, polarimeter is measured the minimum and maximum value of corresponding light signal degree of polarization, and feed back to control module, control module calculates the PDL value of PDL device under test, and according to the required PDL certainty of measurement that reaches, the corresponding computing of controlling the rotary state of polarization rotator at any time and improving the PDL certainty of measurement.And because generally a lot of DWDM device need be measured the PDL value under a plurality of wavelength situations, this can realize by the laser of selecting the wavelength-tunable type in Fig. 1.

The thinking of the PDL measurement scheme of measuring based on DOP derives from PDL influence the research that DOP reaction type PMD compensates.We only provide the required theory of this programme herein.At first, in Jones (Jones) space, set up an x-y coordinate system (the x/y axle corresponds respectively to the fast/slow polarization principal axis of birefringence device), then, as shown in Figure 2, the light signal of input birefringence device is adjusted to 45 ° of linearly polarized lights, and its Jones vector can be written as:

${\stackrel{\→}{E}}_{\mathrm{in}}\left(\mathrm{\ω}\right)=E_{\mathrm{in}}\left(\mathrm{\ω}\right)\·\left(\begin{array}{c}1\\ 1\end{array}\right)---\left(1\right)$

E wherein _InBe that signal is represented at the amplitude of frequency domain (ω).We are the measurement mechanism that example is set forth PDL with the Gaussian transmission signals herein, although our research method is applicable to the light signal of arbitrary shape.Power expression at birefringence device input light signal can be written as:

I _in(t)＝exp(-t ²/2T ²) (2)

Wherein T is the pulse duration of input signal.Therefore, through behind some Fourier transforms, the frequency-domain expression of input signal amplitude can be write:

E _in(ω)＝exp(-ω ²T ²) (3)

Fig. 2 is an input polarization light signal involved in the present invention, the fast/slow main shaft of birefringence device, and the relativeness schematic diagram between the PDL decay axle.Below we provide the Jones matrix of birefringence device and PDL device to be measured respectively.As shown in Figure 2, the Jones matrix of birefringence device can be written as:

$\stackrel{\→}{D}=\left(\begin{array}{cc}{e}^{\mathrm{i\ω}\frac{\mathrm{\Δ\τ}}{2}}& 0\\ 0& e^{-\mathrm{i\ω}\frac{\mathrm{\Δ\τ}}{2}}\end{array}\right)---\left(4\right)$

Wherein Δ τ is the differential group delay (DGD) of birefringence device.Definition according to PDL, because the polarisation-affecting degree is the energy attenuation ratio of signal along two PDL decay axles, so can think equivalently that actual PDL device only has a decay axle, the attenuation coefficient of this decay axle is α (0≤α≤1, α=1 is corresponding to the situation of no PDL), angle between this decay axle and the fast main shaft of birefringence device be β (0＜β≤π), as shown in Figure 2.

The PDL size and the functional relation between attenuation coefficient α of device are as follows:

$\mathrm{PDL}\left(\mathrm{\α}\right)={10\log }_{10}\frac{1}{{\mathrm{\α}}^2}\left(\mathrm{dB}\right)---\left(5\right)$

The Jones matrix of PDL device under test can be written as:

$\stackrel{\→}{\mathrm{\α}}=\left(\begin{array}{cc}\mathrm{\α}{\cos }^2\mathrm{\β}+{\sin }^2\mathrm{\β}& (\mathrm{\α}-1)\sin \mathrm{\β}\cos \mathrm{\β}\\ (\mathrm{\α}-1)\sin \mathrm{\β}\cos \mathrm{\β}& \mathrm{\α}{\sin }^2\mathrm{\β}+{\cos }^2\mathrm{\β}\end{array}\right)---\left(6\right)$

Therefore, the output light signal that detects of polarimeter can be expressed as:

${\stackrel{\→}{E}}_o\left(\mathrm{\ω}\right)=\stackrel{\→}{\mathrm{\α}}\·\stackrel{\→}{D}\·{\stackrel{\→}{E}}_{\mathrm{in}}\left(\mathrm{\ω}\right)=\left(\begin{array}{c}{t}_1\\ t_2\end{array}\right)\·E_{\mathrm{in}}\left(\mathrm{\ω}\right)---\left(7\right)$

T wherein ₁And t ₂To embody formula as follows:

$t_1=(\mathrm{\α}{\cos }^2\mathrm{\β}+{\sin }^2\mathrm{\β})\·e^{\mathrm{i\ω}\frac{\mathrm{\Δ\τ}}{2}}+\frac{\mathrm{\α}-1}{2}\·\sin 2\mathrm{\β}\·e^{-\mathrm{i\ω}\frac{\mathrm{\Δ\τ}}{2}}---(8.a)$

$t_2=\frac{\mathrm{\α}-1}{2}\·\sin 2\mathrm{\β}\·e^{\mathrm{i\ω}\frac{\mathrm{\Δ\τ}}{2}}+(\mathrm{\α}{\sin }^2\mathrm{\β}+{\cos }^2\mathrm{\β})\·e^{-\mathrm{i\ω}\frac{\mathrm{\Δ\τ}}{2}}---(8.b)$

Next we discuss the DOP Changing Pattern under PMD and the PDL acting in conjunction.Following density matrix has been described the polarization state [8] of the light signal that enters polarimeter:

$\mathrm{\ρ}={\∫}_{-x}^{+x}{\stackrel{\→}{E}}_o\left(\mathrm{\ω}\right)\·{\stackrel{\→}{E}}_o^{*}\left(\mathrm{\ω}\right)\mathrm{d\ω}$

$=\frac{1}{2}\left(\begin{array}{cc}{m}_0+m_1& m_2-{\mathrm{im}}_3\\ m_2+{\mathrm{im}}_3& m_0-m_1\end{array}\right)---\left(9\right)$

Wherein " * " represents the conjugate transpose of matrix; m ₀Be the luminous intensity of output signal, and have $m_0=\∫({\stackrel{\→}{E}}_o^{*}\·{\stackrel{\→}{E}}_o)\mathrm{dt};$

Be the Stokes representation of output signal, and have $\stackrel{\→}{m}=\left(\begin{array}{ccc}{m}_1& m_2& m_3\end{array}\right).$

From above (7)-(9) formula, the degree of polarization of output signal can be expressed as:

$\mathrm{DOP}=\frac{\sqrt{m_1^2+m_2^2+m_3^2}}{m_0}---\left(10\right)$

$=\frac{\sqrt{{[({\mathrm{\α}}^2-1)\·\cos 2\mathrm{\β}+\frac{{(\mathrm{\α}-1)}^2}{2}\·\sin 4\mathrm{\β}\·\exp (-\frac{{\mathrm{\Δ\τ}}^2}{{4T}^2})]}^2+{\{({\mathrm{\α}}^2-1)\·\sin 2\mathrm{\β}+[\frac{{(\mathrm{\α}+1)}^2}{2}-\frac{{(\mathrm{\α}-1)}^2}{2}\·\cos 4\mathrm{\β}]\·\exp (-\frac{{\mathrm{\Δ\τ}}^2}{{4T}^2})\}}^2}}{{\mathrm{\α}}^2+1+({\mathrm{\α}}^2-1)\·\sin 2\mathrm{\β}\·\exp (-\frac{\mathrm{\Δ}{\mathrm{\τ}}^2}{{4T}^2})}$

From formula (10) as can be seen: at first, DOP is Δ τ, the function of α and β, and also PDL only just influences the degree of polarization of light signal when PMD is arranged, and this is why we introduce the birefringence device in the PDL measurement mechanism a reason; Secondly, because the PMD value of birefringence device is known determined value among Fig. 1, and the PDL value of PDL device to be measured is also fixing, we just can obtain the PDL value of PDL device to be measured according to the DOP-beta curve property calculation under the different PDL situations that record, such as maximum/minimum value of measuring DOP or DOP slope of a curve or the like.For example, when β=3 π/4 or π/4, we can obtain maximum/minimum value (DOP of DOP respectively _MaxOr DOP _Min), it embodies formula shown in formula (11) and (12):

${\mathrm{DOP}}_{\max }\left(\mathrm{PDL}\right)=\frac{1-{\mathrm{\α}}^2+({\mathrm{\α}}^2+1)\·\exp (-\frac{{\mathrm{\Δ\τ}}^2}{{4T}^2})}{{\mathrm{\α}}^2+1+(1-{\mathrm{\α}}^2)\·\exp (-\frac{{\mathrm{\Δ\τ}}^2}{{4T}^2})}---\left(11\right)$

$=\frac{1-{10}^{-\frac{\mathrm{PDL}}{10}}+({10}^{-\frac{\mathrm{PDL}}{10}}+1)\·\exp (-\frac{{\mathrm{\Δ\τ}}^2}{{4T}^2})}{{10}^{-\frac{\mathrm{PDL}}{10}}+1+(1-{10}^{-\frac{\mathrm{PDL}}{10}})\·\exp (-\frac{{\mathrm{\Δ\τ}}^2}{{4T}^2})}$

${\mathrm{DOP}}_{\min }\left(\mathrm{PDL}\right)=\frac{{\mathrm{\α}}^2-1+({\mathrm{\α}}^2+1)\·\exp (-\frac{{\mathrm{\Δ\τ}}^2}{{4T}^2})}{{\mathrm{\α}}^2+1+({\mathrm{\α}}^2-1)\·\exp (-\frac{{\mathrm{\Δ\τ}}^2}{{4T}^2})}---\left(12\right)$

$=\frac{{10}^{-\frac{\mathrm{PDL}}{10}}-1+({10}^{-\frac{\mathrm{PDL}}{10}}+1)\·\exp (-\frac{{\mathrm{\Δ\τ}}^2}{{4T}^2})}{{10}^{-\frac{\mathrm{PDL}}{10}}+1+({10}^{-\frac{\mathrm{PDL}}{10}}-1)\·\exp (-\frac{{\mathrm{\Δ\τ}}^2}{4T^2})}$

And then can obtain using DOP _MaxAnd DOP _MinThe function expression of the PDL numerical value of the device under test of expression, specific as follows:

$\mathrm{PDL}=5{\log }_{10}\frac{1+{\mathrm{DOP}}_{\max }-{\mathrm{DOP}}_{\min }-{\mathrm{DOP}}_{\max }\·{\mathrm{DOP}}_{\min }}{1+{\mathrm{DOP}}_{\min }-{\mathrm{DOP}}_{\max }-{\mathrm{DOP}}_{\max }\·{\mathrm{DOP}}_{\min }}---\left(13\right)$

By formula (13) and Fig. 1 as can be known, as long as in the rotary course of polarization rotator, accurately record maximum and the minimum value of DOP, just can calculate the PDL value of device under test according to formula (13).

Fig. 3 is the flow chart that PDL involved in the present invention measures feedback control algorithm.At first, the DOP feedback signal of reading system is delivered to the FEEDBACK CONTROL center; Then, send a drive signal by the FEEDBACK CONTROL center, the polarization axle that makes polarization rotator is to changing an angle, promptly (the concrete size of step-length can be decided by the response minimum voltage gradient of control module a step-length, also can be the integral multiple of stepping, specifically want experimental effect and needed certainty of measurement and decide), after polarization rotator responds, send a new DOP feedback signal and deliver to the FEEDBACK CONTROL center, follow the feedback signal value of former storage to compare at once; The polarization axle that makes polarization rotator and so forth is to rotating a circle, in this process, write down DOP maximum and the minimum value that is occurred respectively, and according to formula (13) calculating corresponding devices PDL value, for reducing the PDL measure error, but duplicate measurements n time is (even the polarization axle of polarization rotator is to rotation n week, n 〉=0 is write down the DOP maximum and the minimum value that are occurred in the rotation weekly respectively, and is asked corresponding PDL value) and average (can take compute modes such as weighted average as the case may be).As shown in Figure 3.

By the present invention, we have realized under single wavelength or the multi-wavelength situation PDL feature measurement to fiber optic telecommunications equipment and system.The key of high reliability of the present invention is that the light modulated light signal that enters the birefringence device need be linearly polarized light, and the energy distribution equalization of light signal on these birefringence device two polarization principal axis.For this reason, can adopt the combination of linear polarizer and birefringence device, promptly before the birefringence device, add a linear polarizer (both can be integrated in the practical application), and will adopt polarization maintaining optical fibre to be connected between light source and the linear polarizer.Along with the rotation of polarization rotator, polarimeter is measured corresponding light signal degree of polarization and is fed back to control module, and control module calculates the PDL value of PDL device under test, and controls the rotary state of polarization rotator at any time.The present invention is applicable to that not only the PDL under single wavelength situation of optic communication device or system measures, and is applicable to that equally also the PDL under the multi-wavelength situation measures.

Claims (1)

1, a kind of polarized dependent loss PDL method of measurement is characterized in that: make the incident modulated light signal enter a birefringence device, successively by a polarization rotator, a PDL device under test enters a polarimeter at last then; Make polarization rotator along polarization axle to rotating a circle, in this process, write down DOP maximum and the minimum value that is occurred respectively, and calculate corresponding devices PDL value according to formula (13),

$\mathrm{PDL}=5{\log }_{10}\frac{1+{\mathrm{DOP}}_{\max }-{\mathrm{DOP}}_{\min }-{\mathrm{DOP}}_{\max }\·{\mathrm{DOP}}_{\min }}{1+{\mathrm{DOP}}_{\min }-{\mathrm{DOP}}_{\max }-{\mathrm{DOP}}_{\max }\·{\mathrm{DOP}}_{\min }}---\left(13\right)$

The light signal that enters the birefringence device is a linearly polarized light, the energy distribution equalization of light signal on birefringence device two polarization principal axis.

Images