CN102566196A - Optical time domain differentiator and manufacturing method of such differentiator - Google Patents

Abstract

The invention discloses an optical time domain differentiator and a manufacturing method of such differentiator. The optical time domain differentiator is composed of a circulator and a fiber grating of optical time domain differential. The circulator is a three-port optical device, a second port of the circulator is connected with one end of the fiber grating of optical time domain differential, when a differential signal is input into one end of the fiber grating of optical time domain differential via the second port from the first port of the circulator, a time domain differential signal returned from the fiber grating of optical time domain differential is output from a third port of the circulator. The optical time domain differentiator has the advantages of being simple in structure, easy to integrate and quick in differential realization, being capable of performing time domain differential on high bandwidth signals, and the like.

Description

Optical time domain differentiator and preparation method thereof

Technical field

The present invention relates to differentiator, particularly a kind of optical time domain differentiator and preparation method thereof is mainly used in full light signal and handles and full photometry calculation field.

Background technology

It is in the light territory, directly signal to be handled that full light signal is handled, and has avoided electricity-light and light-electricity conversion, makes it to become the focus in all optical communication research and development.

The optical time domain differentiator can carry out the time domain differential to the optical multiplexed signal light field, is that full light signal is handled a Primary Component of calculating with full photometry.With respect to common electronics differentiator, the optical time domain differentiator can effectively solve processing speed and the bandwidth bottleneck problem that electron device runs into.Except full light signal process field, the optical time domain differentiator can also be used for shaping pulse, the generation of ultrashort pulse sequence, pulse characteristics identification and other fields.

Formerly one of technology is (referring to Xu.J; Et al.; All-optical differentiator based on cross-gain modulation in semiconductor optical amplifier, Opt.Lett.2007.32 (20)) adopts the cross-gain of semiconductor optical amplifier (SOA) to modulate and realize light time territory differential.It is through the synthetic realization of power is carried out in pumping pulse after amplifying and direct impulse.But this optical differential device is a kind of noncoherent time domain differentiator, and it is that light intensity is carried out the time domain differential, rather than the time domain differential is carried out in optics compound light field.Formerly two of technology (referring to Liu.F.F.et.al.; Compact optical temporal differentiator based on silicon microring resonator; Opt.Exp, 2008.16 (20)) use silicon based annular resonant cavity to realize light time territory differential, this silicon based annular resonant cavity is made up of single straight waveguide and a silica-based ring; This device preparation is difficulty comparatively, and cost is higher.

Fiber grating and other optical fibre devices have natural compatibility, do not have coupled problem.And fiber grating has the structural parameters that much can change, and like warbling of the index modulation degree of depth, phase modulation, grating length, change mark shape, grating cycle etc., makes it have very big design flexibility.Owing to avoided employing bulk optics device, therefore stability is high, makes it be widely used in each optical field simultaneously.The present invention proposes a kind of optical time domain differentiator based on fiber grating.

Summary of the invention

The objective of the invention is to overcome the deficiency of above-mentioned prior art, propose a kind of optical time domain differentiator and preparation method thereof, this differentiator has simple in structure, is easy to integratedly, and differential realizes that speed is fast, can carry out the time domain differential to the signal of high bandwidth.

Technical solution of the present invention is following:

A kind of optical time domain differentiator; Characteristics are that its formation comprises the fiber grating of circulator and light time territory differential; Described circulator is a kind of three port optical device; Second port of this circulator connects an end of the fiber grating of territory differential of described light time; Treat that differential signal light gets into an end of the fiber grating of territory differential of described light time by the input of first port of described circulator through second port, the time domain differential signal that returns from the fiber grating of this light time territory differential is from the 3rd port output of this circulator.

The fiber grating of territory differential of described light time is the phase-shifted fiber grating of light time territory differential, the LPFG of light time territory differential, the mole fiber grating of light time territory differential or the apodization fibre-optical raster of light time territory differential.

The preparation method of described optical time domain differentiator, its characteristics are that this method comprises the following steps:

1. the fiber grating for preparing light time territory differential;

2. the end with the fiber grating of territory differential of described light time is connected with second port of described circulator.

The fiber grating of territory differential of described light time is the phase-shifted fiber grating with light time territory differential, follows these steps to prepare:

1. according to the central wavelength lambda of treating differential signal, confirm the periods lambda=λ/n of required phase mask plate _Eff, wherein, n _EffBe the effective refractive index of optical fiber, at first optical fiber carried out the exposure first time and process fiber grating that the index modulation amplitude of optical fiber is Δ n ₁

2. select a re-expose length L in the center of described fiber grating, it is carried out re-expose, making its index modulation amplitude is Δ n ₂, and satisfy:

βΔn ₂L＝π (1)

Wherein: β treats differential signal propagation constant in a vacuum;

Perhaps confirm exposure earlier, confirm that promptly the index modulation amplitude is Δ n ₂, the re-expose length L of definite center position makes it satisfy formula (1) then, and grating is carried out re-expose.

The fiber grating of territory differential of described light time is the phase-shifted fiber grating that utilizes the light time territory differential that blocks the method preparation, and the preparation method comprises the following steps:

1. according to the central wavelength lambda of differential signal to be measured, confirm the periods lambda=λ/n of required phase mask plate _Eff, wherein, n _EffIt is the effective refractive index of optical fiber;

2. confirm the length L of occlusion area, utilize length to shelter from the central area of optical fiber for the shutter of L, utilize the phase-shifted fiber grating of territory differential of disposable preparation light time of phase mask plate, the index modulation amplitude that makes optical fiber is Δ n ₁, and satisfy:

βΔn ₁L＝π (2)

Wherein: β treats differential signal propagation constant in a vacuum;

Perhaps confirm exposure earlier, confirm that promptly the index modulation amplitude is Δ n ₁, the length L of definite center position shutter makes it satisfy formula (2) then.

The fiber grating of territory differential of described light time is the LPFG of light time territory differential, and the preparation method comprises the following steps:

1. according to the central wavelength lambda of differential signal to be measured, confirm the periods lambda=2 λ/(n of required phase mask plate _Core-n _Cladd), wherein, n _CoreBe the effective refractive index of the core layer of optical fiber, n _CladdIt is the effective refractive index of the covering of optical fiber;

2. use the phase mask plate to prepare LPFG; And monitor the reflectance spectrum of LPFG in real time; When the reflectivity of central wavelength lambda reaches maximal value, stop exposure, this moment, the core layer of long period optical fiber can realize optical time domain first order differential function;

3. continue the exposure LPFG, the reflectivity at its central wavelength lambda place is reduced, the long reflectivity of central cardiac wave stops exposure when reaching minimum value, and this moment, the covering of long period optical fiber can be realized optical time domain second-order differential function.

The fiber grating of territory differential of described light time is More's fiber grating of light time territory differential, and the preparation method comprises the following steps:

2. confirm to treat the central wavelength lambda of differential signal and the length L of More's fiber grating;

2. confirm the cycle of two blocks of phase mask plates, make it satisfied:

$\frac{{2\mathrm{\Λ}}_1{\mathrm{\Λ}}_2}{{\mathrm{\Λ}}_1+{\mathrm{\Λ}}_2}=\mathrm{\λ}/n_{\mathrm{Eff}}$ And $\frac{{2\mathrm{\Λ}}_1{\mathrm{\Λ}}_2}{{\mathrm{\Λ}}_1-{\mathrm{\Λ}}_2}=L---\left(3\right)$

Wherein: 2 Λ ₁, 2 Λ ₂Be respectively the cycle of two blocks of phase mask plates, n _EffIt is the effective refractive index of optical fiber;

3. with two blocks of phase mask plates successively twice preparation fiber grating on same optical fiber, and the exposure of twice preparation is identical.

The fiber grating of territory differential of described light time is the apodization fibre-optical raster of light time territory differential, and the preparation method comprises the following steps:

1. according to the central wavelength lambda of differential signal to be measured, confirm the periods lambda=λ/n of required phase mask plate _Eff, wherein, n _EffIt is the effective refractive index of optical fiber;

2. utilize the reverse engineering algorithm computation to go out the apodizing function of apodization fibre-optical raster, and prepare the amplitude template identical with apodizing function;

3. utilize described amplitude template and phase mask plate to prepare required apodization fibre-optical raster.

Principle of work of the present invention:

The realization of optical time domain differentiator of the present invention is based on the derivative characteristic of Fourier transform, its frequency response letter

For: H (ω)=(i (ω-ω _o)) ^k(4)

Wherein: ω _oBe the angular frequency of signal, k is the differential exponent number, and i is an imaginary part.

Principle of the present invention is through phase-shifted fiber grating, and LPFG, fiber gratings such as mole fiber grating and apodization fibre-optical raster obtain that approximate to satisfy the optical time domain differentiator needed suc as formula the frequency response shown in (4).

There is phase transformation in the index modulation of phase-shifted fiber grating; The position of phase variable and the big or small frequency response characteristic that has determined phase-shifted fiber grating; When the phase shift of phase-shifted fiber grating is π and phase shift point when being positioned at the mid point of fiber grating, the spectral response of this structure has the needed frequency response of first order differential device.This phase-shifted fiber grating can and block the method preparation with double exposure:

Described double exposure is promptly: according to the central wavelength lambda of differential signal to be measured, confirm the periods lambda=λ/n of required phase mask plate _Eff, wherein, n _EffIt is the effective refractive index of optical fiber.At first utilize phase mask plate method that optical fiber is carried out the exposure first time, the index modulation amplitude that makes optical fiber is Δ n ₁A fixing re-expose length L is selected in center at fiber grating, and it is carried out re-expose, and making its index modulation amplitude is Δ n ₂, and satisfy formula (1). perhaps confirm exposure earlier, confirm that promptly the index modulation amplitude is Δ n ₂, the re-expose length L of definite center position makes it satisfy formula (1) then, and optical fiber is carried out re-expose. and this moment, phase-shifted fiber grating can be used as single order time domain differentiator.

Describedly block method promptly:, confirm the periods lambda=λ/n of required phase mask plate at first according to the central wavelength lambda of differential signal to be measured _Eff, wherein, n _EffIt is the effective refractive index of optical fiber.Confirm the length L of occlusion area, when making fiber grating, cover the central area of fiber grating, make phase-shifted fiber grating with the method that once becomes grid, the index modulation amplitude that makes fiber grating is Δ n ₁, and satisfy formula (2). perhaps confirm exposure earlier, confirm that promptly the index modulation amplitude is Δ n ₁, the length L of definite center position shutter makes it satisfy formula (2) then.

The core layer of LPFG and the response function of covering can be write as:

$H_{\mathrm{co}}=[\cos \left(\mathrm{\γL}\right)+j\frac{\mathrm{\σ}}{\mathrm{\γ}}\sin \left(\mathrm{\γL}\right)]\exp \left[j({\mathrm{\β}}_{\mathrm{co}}-\mathrm{\σ})L\right]$ (5)

$H_{\mathrm{cl}}=j\frac{\mathrm{\κ}}{\mathrm{\γ}}\sin \left(\mathrm{\γL}\right)\left]\exp \right[j({\mathrm{\β}}_{\mathrm{cl}}+\mathrm{\σ})L]$

H wherein _CoThe response function of expression core layer, H _ClThe response function of expression covering, β _CoAnd β _ClThe propagation constant of expression grating core layer and covering, κ representes coupling coefficient, L representes the length of grating, σ=[β _Co-β _Cl]/2-π/Λ,

Λ is the cycle of grating.Near resonance frequency (being σ → 0), the response function of core layer and covering can be rewritten into:

$H_{\mathrm{co}}\≈[\cos \left(\mathrm{\κL}\right)+j\frac{\mathrm{\σ}}{\mathrm{\κ}}\sin \left(\mathrm{\κL}\right)]\exp \left({\mathrm{j\β}}_{\mathrm{co}}L\right)$ (6)

$H_{\mathrm{cl}}\≈j\{\sin \left(\mathrm{\κL}\right)+\frac{{\mathrm{\σ}}^2}{{2\mathrm{\κ}}^2}[\mathrm{\κ}L\cos \left(\mathrm{\κL}\right)-\sin \left(\mathrm{\κL}\right)\left]\right\}\exp \left[{\mathrm{j\β}}_{\mathrm{cl}}L\right]$

Can find out from formula (6), when κ L=m (pi/2), H _Co∝ j σ ∝ j (ω-ω ₀), the response function of corresponding single order optical time domain differentiator, when κ L=m π, H _Co∝ σ ²∝ (ω-ω ₀) ², the response function of corresponding second order optical time domain differentiator.Therefore when complete coupling condition was satisfied in the resonance coupling of LPFG, promptly satisfy: during κ L=m (pi/2), the fibre core spectral response functions of LPFG satisfied the spectral response demand of single order time domain differentiator.Resonance coupling generation overcoupling when LPFG; And satisfy: during κ L=m π; The covering spectral response functions of LPFG satisfies the spectral response demand of second order time domain differentiator, and this moment, LPFG can be realized the second-order differential to signal.

The mode that can utilize phase mask plate method to write prepares LPFG, at first according to the central wavelength lambda of differential signal to be measured, confirms the periods lambda=2 λ/(n of required phase mask plate _Core-n _Cladd), wherein, n _CoreBe the effective refractive index of the core layer of optical fiber, n _CladdIt is the effective refractive index of the covering of optical fiber.Use the phase mask plate to prepare LPFG, and monitor the reflectance spectrum of LPFG in real time, when the reflectivity of resonance wave strong point reaches maximal value, stop exposure, this moment, the core layer of long period optical fiber can realize optical time domain first order differential function.Continue the exposure LPFG, the reflectivity of its resonance wave strong point is reduced, when the reflectivity at resonance frequency place reaches minimum value, stop exposure, this moment, the covering of long period optical fiber can be realized optical time domain second-order differential function.

More's fiber grating utilizes the Moire effect in the optics, in writing the process of fiber grating, along the index modulation of the axial formation Moire fringe shape of fiber grating, thereby introduces one or several π phase shift point.The number and the position of control phase shift point can make the fiber grating of property, and when only there was a π phase shift point in the mole fiber grating and is positioned at the center of mole fiber grating, its spectral response satisfied the demand of single order optical time domain differentiator.When there was the π phase shift point of two symmetries in the mole fiber grating, its spectral response satisfied the demand of second order optical time domain differentiator, can be used as a second order optical time domain differentiator.

The mole fiber grating of preparation light time territory differential function; At first confirm the central wavelength lambda of differential signal to be measured and the length L of More's fiber grating. select the cycle of two blocks of phase mask plates; Make it to satisfy formula (3). with two blocks of phase mask plates successively twice preparation fiber grating on same optical fiber, and the exposure of twice preparation is identical.

The frequency response function of known fiber optic grating; Can pass through some reverse engineering algorithms, such as a stripping layer algorithm [referring to Skaar, J.et.al.; On the synthesis of fiber Bragg gratings by layer peeling.IEEE J.Quantum Electron.2001.37 (2)]; Can obtain the structural parameters of fiber grating,, just can obtain the index modulation envelope of fiber grating if employed fiber grating is an apodization fibre-optical raster.

Utilize phase mask plate method can prepare the apodization fibre-optical raster of light time territory differential function,, confirm the periods lambda=λ/n of required phase mask plate at first according to the central wavelength lambda of differential signal to be measured _Eff, wherein, n _EffIt is the effective refractive index of optical fiber.Utilize the reverse engineering algorithm computation to go out the apodizing function of apodization fibre-optical raster, and prepare the amplitude template identical with apodizing function. utilize the amplitude template and the phase mask plate of preparation to prepare required apodization fibre-optical raster.

Compared with prior art, the optical time domain differentiator that the present invention is based on fiber grating has following beneficial effect:

1, the optical fibre device of using with optical communication has natural compatibility, does not have coupled problem;

2, avoid adopting the bulk optics device, stability is high;

3, fiber grating can increase the dirigibility of design so that change its structural parameters (like warbling of the index modulation degree of depth, index modulation phase place, grating length, change mark shape, grating cycle etc.);

4, fiber grating is owing to have high signal to noise ratio (S/N ratio), so the spectrum and the time-domain signal of signal has very high signal to noise ratio (S/N ratio) and high resolving power behind the differential.

The present invention has simple in structure, is easy to integratedly, and differential realizes that speed is fast, can carry out advantages such as time domain differential to the signal of high bandwidth.

Description of drawings

Fig. 1 is the structural representation of optical time domain differentiator of the present invention

Fig. 2 is based on the structural representation in optical time domain differentiator when work of More's fiber grating

Fig. 3 is a kind of differential optical signal generator structural drawing of treating

Fig. 4 is the index modulation envelope synoptic diagram of mole fiber grating of the present invention

Fig. 5 is the reflectivity amplitude and the reflected phase will synoptic diagram of mole fiber grating of the present invention

Fig. 6 is the first order differential signal schematic representation of treating differential Gauss light signal and utilizing the mole fiber grating to realize

Fig. 7 is the index modulation envelope synoptic diagram of apodization fibre-optical raster of the present invention

Fig. 8 is the reflectivity amplitude and the reflected phase will synoptic diagram of apodization fibre-optical raster of the present invention

Fig. 9 is the reflectivity amplitude and the reflected phase will synoptic diagram of phase-shifted fiber grating of the present invention

Figure 10 is the core layer reflectivity amplitude and the reflected phase will synoptic diagram of LPFG of the present invention

Figure 11 is the covering response amplitude and the response phase synoptic diagram of LPFG of the present invention

Embodiment

Below in conjunction with embodiment and accompanying drawing the present invention is described further, but should limit protection scope of the present invention with this.

Embodiment one: based on the optical time domain differentiator of mole fiber grating

Fig. 1 is the structural representation of optical time domain differentiator of the present invention; Visible by figure; Optical time domain differentiator of the present invention; Its formation comprises the fiber grating 2 of circulator 1 and light time territory differential, and described circulator 1 is a kind of three port optical device, and second port of this circulator 1 connects an end of the fiber grating 2 of territory differential of described light time; Treat that differential signal light gets into an end of the fiber grating 2 of territory differential of described light time by the input of first port of described circulator 1 through second port, the time domain differential signal that returns from the fiber grating 2 of this light time territory differential is from the 3rd port output of this circulator 1.

In the present embodiment, utilize More's fiber grating to realize the single order time domain differential of light to measured signal, its work structuring synoptic diagram is as shown in Figure 2.In Fig. 2: the 3rd, treat differential signal generator, its schematic configuration diagram is as shown in Figure 3, and the 7th, tunable laser is regulated tunable laser, makes the centre wavelength of continuous laser just in time be positioned at a mole fiber grating resonance valley place.In the present embodiment, the Wavelength of Laser of tunable laser output is 1550.003nm.The output terminal of tunable laser 7 links to each other with the input end of electrooptic modulator 8.The electric signal that electric signal generator 9 produces amplifies the drive signal of back as electrooptic modulator 8.The continuous laser that tunable laser produces is treated differentiated pulse light through what become Gaussian behind the electrooptic modulator.Treat that differential signal generator also can directly replace with pulsed laser.

Treat that the differential signal of treating that differential signal generator 3 produces enters into mole fiber grating 2 through Polarization Controller 4, circulator 1.The index modulation envelope of described mole fiber grating 2 is as shown in Figure 4, and wherein the length of grating is 1 centimetre, and maximum index modulation amplitude is 1*10 ^-4, only there is a π phase shift point in the mole fiber grating this moment and is positioned at the center of grating, its reflection amplitudes and reflected phase will are as shown in Figure 5.Fig. 5 explains that this moment mole fiber grating can satisfy the amplitude and the phase response demand of single order optical time domain differentiator.The signal that comes out from the mole fiber grating detects through detection system 6 after amplifier 5 amplifies, and obtains treating the first order differential signal of differential signal.Fig. 6 is the first order differential signal of importing Gauss pulse and being obtained by the mole grating.

Embodiment two: based on the optical time domain differentiator of apodization fibre-optical raster

Its basic structure is with embodiment one.The index modulation envelope of apodization fibre-optical raster of the present invention is as shown in Figure 7, and its maximum index modulation amplitude is 6.3*10 ^-4, the length of the apodization fibre-optical raster of present embodiment is 5cm.The reflection amplitudes and the phase place of this fiber grating are as shown in Figure 8, and as can be seen from the figure, its frequency response is approximately the needed frequency response of first order differential device, therefore can be used as single order optical time domain differentiator.

Embodiment three: based on the optical time domain differentiator of phase-shifted fiber grating

Utilize phase-shifted fiber grating to realize the time domain differentiator, its basic structure is with embodiment one, and the length of the phase-shifted fiber grating of present embodiment is 2cm, and index modulation is 1*10 ^-4, phase-shift phase is π, and is positioned at the center of fiber grating.Its reflection amplitudes and phase place are as shown in Figure 9, and as can be seen from the figure, its frequency response is approximately the needed frequency response of first order differential device, therefore can be used as single order optical time domain differentiator.Can realize the time domain differentiator of high-order through the cascade phase-shifted fiber grating.

Embodiment four: based on the optical time domain differentiator of LPFG

Utilize LPFG to realize the time domain differentiator; Its basic structure is with embodiment one; Analysis through the front; When complete coupling condition was satisfied in the resonance coupling of LPFG, promptly satisfy: during κ L=m (pi/2), the fibre core spectral response functions of LPFG satisfied the spectral response demand of single order time domain differentiator.Figure 10 is a κ L=pi/2, during grating length 1cm, and the fibre core transmission amplitude and the transmission phase of LPFG, as can be seen from the figure, its frequency response is approximately the needed frequency response of first order differential device, therefore can be used as single order optical time domain differentiator.Figure 11 is κ L=π, when grating length is 2cm, and the covering response amplitude and the response phase of LPFG, as can be seen from the figure, its frequency response is approximately the needed frequency response of second order optical differential device, therefore can be used as second order time domain differentiator.

Claims (8)

1. optical time domain differentiator; Be characterised in that its formation comprises the fiber grating (2) of circulator (1) and light time territory differential; Described circulator (1) is a kind of three port optical device; Second port of this circulator (1) connects an end of the fiber grating (2) of territory differential of described light time; Treat that differential signal light gets into an end of the fiber grating (2) of territory differential of described light time by the input of first port of described circulator (1) through second port, the time domain differential signal that returns from the fiber grating (2) of this light time territory differential is from the 3rd port output of this circulator (1).

2. optical time domain differentiator according to claim 1 is characterized in that the fiber grating (2) of territory differential of described light time is the LPFG of the phase-shifted fiber grating of light time territory differential, light time territory differential, the mole fiber grating of light time territory differential or the apodization fibre-optical raster of light time territory differential.

3. the preparation method of optical time domain differentiator according to claim 1 and 2 is characterized in that this method comprises the following steps:

1. the fiber grating (2) for preparing light time territory differential;

2. the end with the fiber grating (2) of territory differential of described light time is connected with second port of described circulator (1).

4. the preparation method of optical time domain differentiator according to claim 3 is characterized in that the fiber grating (2) of territory differential of described light time is the phase-shifted fiber grating with light time territory differential, utilizes optical fiber to follow these steps to prepare:

1. according to the central wavelength lambda of treating differential signal, confirm the periods lambda=λ/n of required phase mask plate _Eff, wherein, n _EffBe the effective refractive index of optical fiber, at first optical fiber carried out the exposure first time and process fiber grating that the index modulation amplitude of optical fiber is Δ n ₁

2. select a re-expose length L in the center of described fiber grating, it is carried out re-expose, making its index modulation amplitude is Δ n ₂, and satisfy:

βΔn ₂L＝π

Wherein: β treats differential signal propagation constant in a vacuum;

Perhaps confirm exposure earlier, confirm that promptly the index modulation amplitude is Δ n ₂, confirm then the re-expose length L of center position it to be satisfied:

βΔn ₂L＝π

Grating is carried out re-expose.

5. the preparation method of optical time domain differentiator according to claim 3, the fiber grating (2) that it is characterized in that territory differential of described light time is the phase-shifted fiber grating that utilizes the light time territory differential that blocks the method preparation, the preparation method comprises the following steps:

1. according to the central wavelength lambda of differential signal to be measured, confirm the periods lambda=λ/n of required phase mask plate _Eff, wherein, n _EffIt is the effective refractive index of optical fiber;

2. confirm the length L of occlusion area, utilize length to shelter from the central area of optical fiber for the shutter of L, utilize the phase-shifted fiber grating of territory differential of disposable preparation light time of phase mask plate, the index modulation amplitude that makes optical fiber is Δ n ₁, and satisfy:

βΔn ₁L＝π

Wherein: β treats differential signal propagation constant in a vacuum;

Perhaps confirm exposure earlier, confirm that promptly the index modulation amplitude is Δ n ₁, confirm then the length L of center position shutter it to be satisfied:

βΔn ₁L＝π

6. the preparation method of optical time domain differentiator according to claim 3 is characterized in that the fiber grating (2) of territory differential of described light time is the LPFG of light time territory differential, and the preparation method comprises the following steps:

1. according to the central wavelength lambda of differential signal to be measured, confirm the periods lambda=2 λ/(n of required phase mask plate _Core-n _Cladd), wherein, n _CoreBe the effective refractive index of the core layer of optical fiber, n _CladdIt is the effective refractive index of the covering of optical fiber;

2. use the phase mask plate to prepare LPFG, and monitor the reflectance spectrum of LPFG in real time, when the reflectivity of central wavelength lambda reaches maximal value, stop exposure, this moment, the fibre core of long period optical fiber can be realized optical time domain first order differential function;

3. continue the exposure LPFG, the reflectivity at its central wavelength lambda place is reduced, the long reflectivity of central cardiac wave stops exposure when reaching minimum value, and this moment, the covering of long period optical fiber can be realized optical time domain second-order differential function.

7. the preparation method of optical time domain differentiator according to claim 3 is characterized in that the fiber grating (2) of territory differential of described light time is More's fiber grating of light time territory differential, and the preparation method comprises the following steps:

1. confirm to treat the central wavelength lambda of differential signal and the length L of More's fiber grating;

2. confirm the cycle of two blocks of phase mask plates, make it satisfied:

$\frac{{2\mathrm{\Λ}}_1{\mathrm{\Λ}}_2}{{\mathrm{\Λ}}_1+{\mathrm{\Λ}}_2}=\mathrm{\λ}/n_{\mathrm{Eff}}$ And $\frac{{2\mathrm{\Λ}}_1{\mathrm{\Λ}}_2}{{\mathrm{\Λ}}_1-{\mathrm{\Λ}}_2}=L$

Wherein: 2 Λ ₁, 2 Λ ₂Be respectively the cycle of two blocks of phase mask plates, n _EffIt is the effective refractive index of optical fiber;

3. with two blocks of phase mask plates successively twice preparation fiber grating on same optical fiber, and the exposure of twice preparation is identical.

8. the preparation method of optical time domain differentiator according to claim 3 is characterized in that the fiber grating (2) of territory differential of described light time is the apodization fibre-optical raster of light time territory differential, and the preparation method comprises the following steps:

1. according to the central wavelength lambda of differential signal to be measured, confirm the periods lambda=λ/n of required phase mask plate _Eff, wherein, n _EffIt is the effective refractive index of optical fiber;

2. utilize the reverse engineering algorithm computation to go out the apodizing function of apodization fibre-optical raster, and prepare the amplitude template identical with apodizing function;

3. utilize described amplitude template and phase mask plate to prepare required apodization fibre-optical raster.

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