CN1232867C - Adjustable filter based on optical fiber supersonic super structure grating and modulating method - Google Patents

Abstract

The present invention relates to a tunable filter based on optical fiber supersonic super structure grating and modulating method. The present invention is characterized in that the tunable filter comprises an optical fiber grating generating reflection spectrum lines, an ultrasonic radiator, a concentrator, an ultrasonic absorber and a support frame, wherein the concentrator is arranged between the ultrasonic radiator and the optical fiber grating; the ultrasonic radiator is stuck to the surface of the large end face of the concentrator; the concentrator forms a bellmouth shape; the other end of the optical fiber is provided with the ultrasonic absorber. The reflection spectrum of the optical fiber ultrasonic fiber grating with a superstructure has a plurality of reflection peaks; the wavelength interval of every two reflection peaks is decided by ultrasonic frequency; the reflectivity can be regulated by ultrasonic power. Thereby, the ultrasonic frequency and power can be changed; the reflection wavelength and reflectivity of one stage reflection spectrum is regulated; thereby, a restructurable demultiplexing device of an upper and a lower optical paths for monitoring the power of an optical channel can be realized. The present invention has the advantages of compact structure, quick tuning and wide tuning range; no stress is applied to the optical fiber grating on the axial direction; the service life is extended.

Description

A kind of adjustable filter and modulator approach based on fiber ultrasonic superstructure grating

Technical field

The present invention relates to a kind of adjustable filter and modulator approach based on fiber ultrasonic superstructure grating, it is that a kind of ultrasonic grating that uses high-frequency ultrasonic to produce is modulated fiber grating more precisely, thereby the drift that produces wavelength realizes tunable filtering.Belong to optical communication field.

Background technology

Fiber grating is by in the length of several millimeters of single-mode fiber cores or several centimetres, by ultraviolet exposure form fiber core refractive index periodically subtle change form.In recent years, increasingly mature along with the preparing grating technology, fiber grating has obtained widespread use at optical communication field, becomes one of important devices in this field.Aspect optical communication, it can constitute Primary Components such as distributed feed-back formula laser instrument, light wavelength division multiplexing, dispersion compensator, wave filter; Aspect Fibre Optical Sensor, fiber grating plays an increasingly important role in water power dam, bridge, the isostructural real-time monitoring of highway owing to its inherent advantage.

Generally can regulate by the spacing of change grid or the effective refractive index realization tunable optic filter of grating region by methods such as temperature and stress.Concrete tuning methods has:

1, people such as the tuning Ball of electroceramics (PZT) driving study adjustable optic fibre laser instrument (Ball G A, Morey W W.Continuously tunable single-mode erbium fiber laser.Opt.lett., 1992,17 (6): 420-422), constitute the resonator cavity of laser instrument as catoptron with a pair of FBG of identical operation wavelength, and the two ends in chamber are fixed on (PZT) on the piezoelectric ceramic actuator, with stretching of Control of Voltage PZT, reach and regulate the long and FBG wavelength in chamber, adjustment process is continuous, but non-linear this tuning methods that determined of the electrostrictive effect of piezoelectric ceramics is non-linear tuning, and the drift that the hysteresis effect of piezoelectric ceramics can produce wavelength maybe can't be made zero.The deformation quantity of piezoelectric ceramics is limited during week, has limited tuning range.

2, electromagnetic force drives that tuning iron core in the middle of making is inhaled mutually or repelled each other by sense of current in the control solenoid, is compressed or stretches thereby make two be fixed on grating on the support, reaches tunning effect.(Zhao Ling, Cai Haiwen etc., fiber grating electromagnetic force tuning Study on Technology. Chinese laser .2002.29 (4) .313-316 and application for a patent for invention publication number CN 1338649A, the bidirectional stress tuner of fiber grating) according to the principle of magnetic tuning, designed mechanical tuning device as shown in Figure 1, problem at preventing the crooked deformation of fiber grating in compression-tuned has designed special-purpose guide rail mechanism, and proof stress is axial along fiber grating.Fiber grating with the withered knot of epoxy resin with support that the magnetic core of solenoid is connected on, support links to each other with base plate.Can be by sense of current in the control solenoid, thus make magnetic core in the coil inhale mutually or repel each other. make support produce relative motion. fiber grating is subjected to axial compression stress or tension, realizes " two-way adjustable ".

3, the upper surface of the tuning semi-girder that sticks on uniform dielectric with the fiber grating rigidity of semi-girder, its direction is consistent with two axis, and grating and stiff end have certain intervals.(Yu Youlong, Liu Zhiguo, Dong Xiaoyi etc., based on the fiber grating of semi-girder do not have warble linear tuning, optics journal Vol.19, No.5, May, 1999) this method is to utilize the cycle of the deformation change fiber grating of semi-girder, but because need sticking on the semi-girder with the fiber grating rigidity, certainly will cause the influence of the bond stress of glue to the optical grating reflection spectral line, the cycle that causes fiber grating simultaneously can not even variation in crooked in semi-girder is subjected to, and therefore chirp phenomenon can occur in tuning process.Yet the generation of avoiding this phenomenon then needs the two ends of a fixed grating, can only realize unidirectional tuningly like this, has reduced tuning range more.

Above-mentioned prior art realizes that the tuning methods of adjustable filter can reduce that stress is tuning vertically, in stress is tuning, draw ratio compression to optical fiber is more prone to, but implement compression and can reach bigger tuning range, and compression strenght is far longer than pulling strengrth, but the compression of optical fiber is difficult to avoid the fibre-optical bending problem.No matter be to stretch or compression, optical fiber all there is damage, the reliability of optical fiber and life-span are that this tuning methods needs to pay close attention to more, simultaneously owing to be the tuning of employing power, device package is required to introduce extra-stress, otherwise can bring centre wavelength drift, warble, problem such as wavelength can't be replied.

Summary of the invention

The invention provides a kind of based on fiber ultrasonic superstructure adjustable filter and modulator approach.It can wide region, fast, multi-wavelength is tuning, do not need to adopt mechanical clamp to fiber grating axis direction stress application, overcome above-mentioned prior art fiber grating is implemented stress shortcoming to the damage of grating when tuning, thereby avoided owing to the damage that stretches or compression causes optical fiber, thereby increased the service life.

When ultrasound wave is propagated in medium, to make density of medium produce small periodic disturbance, thereby cause the refractive index of medium that small cyclical variation takes place, the cyclical variation of this refractive index can be thought a series of catoptrons in space with velocity of sound motion, when light is propagated in medium, by these mirror reflects, caused the variation of phase cycling, therefore be referred to as ultrasonic grating.It is a kind of phase grating.Change the characteristic that frequency of ultrasonic and power can change grating fast.

When light signal was propagated in optical fiber, optical fiber was the light wave transmissions medium, was again the sonic transmissions medium.When with the ultrasound wave parallel boot on optical fiber the time, hyperacoustic mode of propagation is ultrasonic travelling wave (dilatational wave), the molecule that is optical fiber is according to compression and the stretching mutually of certain intervals distance, and the distance between two compressions (or stretching) are located is decided by the wavelength of the sound wave in the optical fiber.Therefore, the density of optical fiber and refractive index are all done the cycle variation along the direction of ultrasonic propagation, thereby constitute ultrasonic fiber grating, and diffraction will take place the light signal that transmits in this medium, as diffraction grating.Owing to what produce at optical fiber is the row ripple, i.e. extensional vibration, if make fiber grating on this section optical fiber, sound wave will produce periodic the modulation to fiber grating.Ultrasonic travelling wave is determined by following formula along the velocity of propagation of optical fiber:

$c_s=\sqrt{\frac{E}{\mathrm{\ρ}}}---\left(1\right)$

Wherein E is that the Young modulus of optical fiber is 7 * 10 ¹⁰N/m ², ρ is the density of quartz glass.

When ultrasound wave is propagated in optical fiber or optical waveguide, form ultrasonic grating, ultrasonic grating is modulated at the light signal of wherein propagating, and two co-propagate patterns of optical fiber or optical waveguide are intercoupled, realized full optical fiber acousto-optic tunable filter, based on the acousto-optic tunable filter of LiNbO3.But this wave filter can only be realized the coupling of co-propagate pattern, and its work zone is wide, and device size is big.

Fiber ultrasonic superstructure grating be by ultrasound wave produce with the ultrasonic grating of cosine distribution result to optical fiber Bragg (Bragg) Grating Modulation.The ultrasonic grating function distributes:

F(z)＝F ₀×cos(k _s×z) (2)

K wherein _sBe hyperacoustic transmission, k _s=2 π/Λ _s, Λ _sBe the cycle of sound wave, F ₀Be hyperacoustic amplitude, this amplitude is the function of hyperacoustic speed and power, $F_0=\sqrt{{2P}_s/\left({\mathrm{EAv}}_s\right)},$ P in the formula _sBe acoustical power, E is the Young modulus of optical fiber, and A is the sectional area of fiber grating, v _sBe the speed that ultrasound wave is propagated in medium.The longitudinal wave that produces when ultrasound wave transmits on optical fiber is to producing the compression and the stretching in cycle along optical axis direction, make the phase place of fiber grating produce periodic modulation, form fiber ultrasonic superstructure grating, its reflectance spectrum and sampled-grating are similar, be periodic a plurality of reflection peak (Fig. 3 a, b, c), it reflects peak-to-peak interval and depends on hyperacoustic periods lambda _s, and ultrasonic wave system q, the pass is between them:

$q=\frac{{\mathrm{\Λ}}_s}{{\mathrm{\Λ}}_{\mathrm{bragg}}}---\left(3\right)$

Λ in the formula _BraggIt is the cycle of optical fiber Bragg (Bragg) grating.Because ultrasound wave will periodically change the periods lambda of Bragg grating _BraggThereby, periodically changed the phase place of fiber grating, form the superstructure grating.Therefore, fiber ultrasonic superstructure grating index distribution function n _Eff(z) can be expressed as:

n _eff(z)＝ δn _eff(z)[1+vcos(t·z+α·cos(k _s·z))] (4)

δ n _Eff(z) being the index modulation degree of depth, is constant for even Fiber Bragg Grating FBG, and v is the visible coefficient of refractive index, general v≤1, and the function part after the modulation has J according to the BESSEL functional expansion ₀(α) and J ₁(α) ... J _n(α) item:

$\cos (t\·z+a\·\cos \left(k_{s}z\right))=J_0\left(a\right)\·\cos (t\·z)+\underset{n=1}{\overset{\∞}{\mathrm{\Σ}}}{J}_n\left(a\right)\·[\cos (t\·z+n\·k_s\·z)+{(-1)}^n\·\cos (t\·z-n\·k_s\·z)]$

(5)

Know the reflection spectral line of fiber ultrasonic superstructure grating thus, centre wavelength is that the Bragg wavelength location immobilizes reflection coefficient by zeroth order BESSEL function J ₀(α) decision.Single order BESSEL function J ₁The position of reflection spectral line (α) is by the coefficient q decision (as Fig. 3) in ultrasonic grating and fiber grating cycle, and reflection coefficient is by J ₁(α) decision.Definition factor alpha=tF ₀/ k _s, $t=\frac{2\mathrm{\π}}{{\mathrm{\Λ}}_{\mathrm{bragg}}},$ $k_s=\frac{2\mathrm{\π}}{{\mathrm{\Λ}}_s}.$ Λ wherein _BraggBe the cycle of Bragg fiber grating, Λ _sIt is hyperacoustic cycle.λ is to use wavelength, λ _BraggIt is the reflection wavelength of Bragg grating.Z is along the Bragg grating position distribution.What the variation of phase place had changed grating counts σ from coupled systemes, thereby has changed the reflectivity of fiber grating.The counting σ from coupled systemes and be of fiber ultrasonic superstructure grating:

$\mathrm{\σ}=2\mathrm{\π}\·n_{\mathrm{eff}}(\frac{1}{\mathrm{\λ}}-\frac{1}{{\mathrm{\λ}}_{\mathrm{bragg}}})+\frac{2\mathrm{\π}}{\mathrm{\λ}}\·\stackrel{\‾}{\mathrm{\δ}}{n}_{\mathrm{eff}}-\frac{1}{2}{k}_s\·a\·\sin (k_s\·z)---\left(6\right)$

Therefore, as long as change ultrasonic coefficient q,, just can change single order reflection spectral line J promptly by selecting hyperacoustic wavelength ₁Position (α), thus the reflection spectral line of different interval obtained, realized the dynamic tuning of wavelength.And can on optical fiber, load the ultrasound wave of a plurality of different wave lengths simultaneously, realize that a plurality of wavelength are simultaneously tuning.

Tunable optic filter provided by the invention is a kind of acousto-optic tunable filter of realizing based on the ultrasonic transducer of piezoelectric.Comprise the fiber grating that produces the reflection spectral line, ultrasonic radiator, amplitude transformer, ULTRASONIC ABSORPTION device, and support.Because high-frequency ultrasonic writes on the fiber grating in ultraviolet and propagates, the long period ultrasonic grating modulation optical fiber optical grating of generation forms fiber ultrasonic superstructure grating.There are a plurality of reflection peaks in the reflection spectral line of fiber ultrasonic superstructure grating, and the wavelength interval of its reflection peak is determined that by frequency of ultrasonic reflectivity is by hyperacoustic power adjustments.Therefore, can regulate the wavelength and the reflectivity of single order reflectance spectrum, thereby can realize downloading multiplexer on the power monitoring of optical channel and the reconfigurable light by changing frequency of ultrasonic and power.In order to make full use of the energy of high-frequency ultrasonic, between radiator and fiber grating, amplitude transformer has been installed, this amplitude transformer is a kind of bar of hydraucone shape, the end surfaces bonding that the area of ultrasonic radiator and amplitude transformer is big, thereby the ultrasound wave that ultrasonic transducer sends is gathered on the optical fiber, increase the Oscillation Amplitude of optical fiber.The bell-mouthed xsect of amplitude transformer reduces gradually, and the amplitude of particle wherein and speed will be along with bell-mouthed contraction increasing (as Fig. 2), and the contraction rate of general loudspeaker is measured with β,

M＝M ₀·e ^-2β·l

M ₀Be the initial area of ultrasonic radiator, l is the distance apart from primary face.Horn length should be installed in such ultrasonic radiator on the fiber grating for the integral multiple of the half-wavelength of ultrasonic radiation modulator material natural frequency, and at the other end of grating the ULTRASONIC ABSORPTION device is installed.

In order to improve the energy transmission efficiency between ultrasound wave and the optical fiber, making the hydraucone amplitude transformer is outside the requisite measure, also need to take two measures: the one, with the glue that uses Shore hardness＞75 that is connected between hydraucone and the optical fiber, and glue thickness is thin as far as possible, usually the aperture in the hydraucone than the big 1～3um of fiber core about, the 2nd, this section optical fiber need be peelled off surface coating layer in the hydraucone amplitude transformer.

In sum: the invention is characterized in that the high-frequency ultrasonic by piezoelectric is produced is loaded on the optical fiber that is carved with grating, by the wavelength of high-frequency ultrasonic or the variation of frequency, the dynamic tuning that cardiac wave becomes in the realization; Adopt high-frequency ultrasonic to modulate the fiber ultrasonic superstructure grating of formation at the uniform fiber grating that the ultrasonic grating that produces on the optical fiber is carved with on to optical fiber, by changing hyperacoustic cycle, change the position of single order spectral line, realize the dynamic tuning of wavelength, obtain the reflection spectral line of different interval; Ultrasound wave transmits this grating or reflective bragg grating, or supercycle fiber grating being carved with on the optical fiber of grating.

Fiber ultrasonic superstructure grating adjustable filter provided by the invention, its advantage is conspicuous:

1) tuned speed faster, wideer tuning range;

2) owing to do not need to adopt mechanical clamp to fiber grating axis direction stress application,, thus avoided since stretch or compression to the damage that optical fiber causes, prolonged serviceable life.

3) compact conformation.

Description of drawings

Fig. 1 fiber ultrasonic superstructure grating adjustable filter of the present invention structural drawing

101 ultrasonic radiators, 102 amplitude transformers, 103 optical fiber, 104 fiber gratings, 105 supports, 106 ULTRASONIC ABSORPTION devices, 107 fillers, 108 signal sources.

The connected mode synoptic diagram of Fig. 2 hydraucone and optical fiber

The reflection spectral line of the ultrasonic superstructure fiber grating of Fig. 3

(a) be α=0.5, q=50, L=20mm, δ n _Eff(z)=5 * 10 ^-4The time the reflection spectral line

(b) be α=0.5, q=100, L=20mm, δ n _Eff(z)=5 * 10 ^-4The time the reflection spectral line

(c) be α=0.5, q=100, L=20mm, δ n _Eff(z)=5 * 10 ^-4The time, the detail view of single order reflection spectral line

301 is single order BESSEL function J ₁Reflection spectral line (α), 302 is centre wavelength, 303 is the secondary spectral line.

Download multiplexer (OADM) on the light of Fig. 4 based on the fiber ultrasonic superstructure grating adjustable filter

Embodiment

Further specify substantive distinguishing features of the present invention and obvious improvement below by embodiment, with and as on the light/download the application of multiplexer:

A kind of as shown in Figure 1 acousto-optic tunable filter of realizing based on the ultrasonic transducer of piezoelectric (AOTF) of embodiment 1.In order to make full use of hyperacoustic energy, between radiator 101 and fiber grating 104, amplitude transformer 102 has been installed, i.e. a kind of bar of horn shape, the end surfaces that radiator 101 and amplitude transformer 102 areas are big is bonding, and using Shore hardness is 80 glue.Thereby the ultrasound wave that ultrasonic transducer sends is gathered on the optical fiber 103, thereby increase the Oscillation Amplitude of optical fiber.And for example shown in Figure 2, the aperture 201 in the hydraucone is than the big 2um of diameter of optical fiber 103, and this section optical fiber need be peelled off surface coating layer in the hydraucone amplitude transformer, to improve the energy transmission efficiency between ultrasound wave and the optical fiber.

Embodiment 2 its structures as shown in Figure 4.On the light based on fiber ultrasonic superstructure adjustable filter/download the application of multiplexer.It is to have adopted ultrasound wave is loaded into scheme on the Fiber Bragg Grating FBG, therefore connected circulator 402,405 in the front and back of adjustable filter 404, when the communication wavelengths that enters from input channel 401 is passed through tunable optic filter 404, by tuning hyperacoustic cycle (being wavelength or frequency), necessary wavelength is reflected, and download from the download channel 403 of circulator 402; Simultaneously can necessary wavelength be uploaded to output channel 407 by uploading passage 406 by circulator 405, thereby realize on the dynamic optical wavelength/download the function of multiplexer, and simultaneously its up and down carrier wave length can change according to the change of fiber ultrasonic superstructure grating adjustable filter institute tuning wavelength, realized reconfigurable light top and bottom path multiplexer (ROADM).

Claims (6)

1. the acousto-optic tunable filter based on fiber ultrasonic superstructure grating opens and is characterised in that it is configured to by producing fiber grating, ultrasonic radiator, amplitude transformer, ULTRASONIC ABSORPTION device, the acoustic absorption material that reflects spectral line and propping up; Between ultrasonic radiator and fiber grating, amplitude transformer is installed, the end surfaces bonding that ultrasonic radiator and amplitude transformer area are big; The other end of fiber grating is installed the ULTRASONIC ABSORPTION device.

2. by the described a kind of acousto-optic tunable filter of claim 1 based on fiber ultrasonic superstructure grating, it is characterized in that described amplitude transformer is a kind of bar of hydraucone shape, bell-mouthed xsect reduces gradually, and the amplitude of particle wherein and speed will be increasing with bell-mouthed contraction.

3. by claim 1 or 2 described a kind of acousto-optic tunable filters, it is characterized in that the glue that uses Shore hardness＞75 that is connected between described amplitude transformer hydraucone and the optical fiber based on fiber ultrasonic superstructure grating; Aperture in the hydraucone is than the big 1～3um of fiber core; Optical fiber in the amplitude transformer need be peelled off surface coating layer.

4. modulator approach based on the acousto-optic tunable filter of fiber ultrasonic superstructure grating, it is characterized in that adopting high-frequency ultrasonic to be carved with uniform grating on to optical fiber and modulate the formation fiber ultrasonic superstructure grating at the ultrasonic grating that produces on the optical fiber, by changing hyperacoustic cycle, change the position of single order reflection spectral line, thereby realize the dynamic tuning of wavelength, obtain the reflection spectral line of different interval.

5. by the modulator approach of the described a kind of acousto-optic tunable filter based on fiber ultrasonic superstructure grating of claim 4, it is characterized in that hyperacoustic periods lambda is depended at the interval of described reflection spectral line _s, and ultrasound wave coefficient q, the pass between them is $q=\frac{{\mathrm{\Λ}}_s}{{\mathrm{\Λ}}_{\mathrm{bragg}}},$ Λ in the formula _BraggIt is the cycle of optical fiber Bragg (bragg) grating.

6. by claim 4 described a kind of acousto-optic tunable filter and modulator approach based on fiber ultrasonic superstructure grating, it is characterized in that described fiber ultrasonic superstructure grating is that high-frequency ultrasonic is propagated on the fiber grating that ultraviolet ray writes, the long period ultrasonic grating modulation optical fiber optical grating of generation forms.

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