CN1420374A - Low return loss etching diffraction grating wave-length division multiplexer - Google Patents

Abstract

A wave length division multiplexer with etched diffraction raster and low return loss is composed of input waveguide, output waveguide, etched raster, and free propagation region. The incident point and outgoing point of input and output waveguides are positioned on a circle. The center of said etched raster is positioned at a tangential point between said circle and another circle. The direction of each reflecting tooth surface for said raster is determined by incident and outgoing directions to reflect the incident light to same position where there is no output astigmation, resulting in minimal optical return loss.

Description

Low return loss etching diffraction grating wavelength division multiplexer

Technical field

The present invention relates to optical communication wavelength-division multiplex field, particularly a kind of low return loss etching diffraction grating wavelength division multiplexer.

Background technology

Wavelength-division multiplex (WDM) technology can to satisfy the demand to bandwidth that increases, be one of effective ways that improve communication capacity at the multiplexing dozens or even hundreds of wavelength of simple optical fiber.Wavelength-division multiplex technique has now improved to such an extent that can transmit the laser of hundreds of wavelength, interval 1.6,0.8 or 0.4nm between each wavelength, even 0.2nm, and this technology is called dense wave division multipurpose (DWDM).WDM has been widely used in the long-distance transmission, also begins to enter in the short distance optical communication network.

Wavelength division multiplexer is the device of most critical in the division multiplex fibre-optic communication wave system.Existing realization technology mainly contains film filtering, fiber grating, integrated planar waveguide etc.Wherein integrated planar waveguide wavelength division multiplexer can be in the dense wave division multipurpose that realizes on the very little chip more than 40 passages, and it utilizes semiconductor technology to produce in enormous quantities, has potential cost advantage.Most typical integrated planar waveguide wavelength division multiplexer is array waveguide grating (AWG), its technology comparative maturity, and the product of 40 passages has been realized commercialization.Another kind is based on the integrated planar waveguide wavelength division multiplexer of etched diffraction grating (EDG), and it has littler size, can realize the single face encapsulation, and be easy to realize the more wavelength-division multiplex of high channel number.Although the EDG manufacture craft requires than AWG height, it is still very potential a kind of integrated planar waveguide wavelength division multiplexer.

Return loss is an important indicator of optical device, and it is meant luminous energy that reflects back into incident path in the light path and the ratio of going into firing association's luminous energy, represents with dB.Return loss has weakened the performance of laser instrument, and the relevant quality that has influence on light signal of the repeatedly catadioptric of light in light path, and noise is increased, and the bit error rate increases.The return loss of general optical device derives from end face reflection (Fresnel reflection) and material backscattering (Rayleigh backscattering).And be a kind of reflection grating type device based on the integrated planar waveguide wavelength division multiplexer of etched diffraction grating, thereby the source of return loss also has optical grating reflection except that end face reflection and material scattering.The coupling light in each output waveguide of most of wavelength in etching diffraction grating wavelength division multiplexer, input waveguide only may receive the light of particular range of wavelengths.If input light does not comprise these wavelength, grating reflection just can not cause return loss.Yet because the influence of environment such as temperature, the incident light wavelength can produce drift, if the drift of wavelength reaches the wavelength that input waveguide can receive, will produce return loss.In addition, the ground unrest that is used for the EDFA of light signal amplification in the optical communication system has very wide frequency spectrum, and these noise light are got back to the incident waveguide, will produce return loss.

The general method that reduces return loss is the interface of adopting the angle of inclination, but this method can only reduce the return loss that cause at the interface, and the return loss that other factors such as optical grating reflection are caused is invalid.In light path, use the return loss that reduces that optoisolator can be whole, but cost and complexity that the use of optoisolator has increased, and increased extra insertion loss.Etching diffraction grating wavelength division multiplexer is because its intrinsic input and output position is very approaching, and the source that causes the device return loss mainly is a grating reflection.The present invention proposes the method for designing of making low return loss etching diffraction grating wavelength division multiplexer.This method is compared with the common method that reduces return loss (as using optoisolator) when improving the return loss performance to greatest extent, additional any geometry, and manufacture craft does not change yet, and realization is simple, and cost does not increase.

Summary of the invention

The purpose of this invention is to provide a kind of low return loss etching diffraction grating wavelength division multiplexer, guarantee that input waveguide receives minimum light return loss.

The technical solution used in the present invention is as follows:

It comprises input waveguide, output waveguide, etched grating, free propagation region.The incidence point of input waveguide and output waveguide and eye point are positioned on the circle of same radius, and etched grating to be positioned at 2 times of these radius of circles be radius, and therewith on another circle of round phase inscribe, the center of etched grating is on the point of contacts of two circles.Each reflection flank of tooth of etched grating towards all determining by incident direction and exit direction, make incident light reflex to same output aberrationless point position.

Incidence point position that said input waveguide is determined and the satisfied θ that concerns in output aberrationless point position _D0=θ _i-arcsin (2tg θ _i/ m), make the minimum value place of incidence point position at diffraction light distribution envelope; θ in the formula _D0Be the angle of diffraction of output aberrationless point position with respect to etched grating, θ _iBe the incident angle of incidence point position with respect to etched grating, m is the grating diffration exponent number.

The present invention adopts ripe semiconductor deposition and etching technics, obtain single-mode plate waveguide (for example the method with plasma enhanced chemical vapor deposition is made three layers of silicon dioxide optical waveguide on silicon base) in deposition on the semiconductor base material, light can obtain (for example using the inductively coupled plasma etching silicon dioxide) by dry etching when input and output waveguide and diffraction grating in the sandwich layer single mode transport of waveguide.

Advantage of the present invention is: it is compared with the common method that reduces return loss (as using optoisolator) when improving the return loss performance to greatest extent, additional any geometry, and manufacture craft does not change yet, and realization is simple, and cost does not increase.

Description of drawings

Fig. 1 is the etching diffraction grating wavelength division multiplexer structure;

Fig. 2 is low return loss etching diffraction grating wavelength division multiplexer grating flank of tooth design concept enlarged drawing;

Fig. 3 is low return loss etching diffraction grating wavelength division multiplexer optical field distribution in output face when the incident of different wave length input light;

Fig. 4 is the etching diffraction grating wavelength division multiplexer of low return loss design and the comparison that two kinds are not adopted the etching diffraction grating wavelength division multiplexer of low return loss design (gathering the design of incidence point and the design of the low return loss of common nothing) in input waveguide place optical field distribution;

Fig. 5 is the etching diffraction grating wavelength division multiplexer of low return loss design and the comparison that two kinds are not adopted the etching diffraction grating wavelength division multiplexer of low return loss design (gathering the design of incidence point and the design of the low return loss of common nothing) in the response of input waveguide intermediate frequency spectrum.

Embodiment

Low return loss etching diffraction grating wavelength division multiplexer as shown in Figure 1 comprises input waveguide 1, output waveguide 2, etched grating 3, free propagation region 4.The incidence point of input waveguide 1 and output waveguide 2 and eye point are positioned on the round A of same radius, and etched grating 3 to be positioned at 2 times of round A radiuses be radius, and with circle A mutually on the round B of inscribe, the center of etched grating is on two point of contacts of justifying.As shown in Figure 2, each reflection flank of tooth 7 of etched grating towards all determining by incident direction and exit direction, make incident light reflex to same output aberrationless point position 6.

Following relation: θ is satisfied in incidence point position 5 that input waveguide is determined and output aberrationless point position 6 _D0=θ _i-arcsin (2tg θ _i/ m), make the minimum value place of incidence point position 5 at diffraction light distribution envelope; θ in the formula _D0Be the angle of diffraction of output aberrationless point position with respect to etched grating, θ _iBe the incident angle of incidence point position with respect to etched grating, m is the grating diffration exponent number.

Low return loss etching diffraction grating wavelength division multiplexer spare of the present invention adopts semiconductor technology to plate film with PECVD (plasma enhanced chemical vapor deposition) or FHD technologies such as (flame water oxygen are sent out) on silicon chip or other base materials, forms planar waveguide.Input and output waveguide and diffraction grating adopt RIE (reactive ion etching) or ICP methods such as (inductively coupled plasma etchings) to carry out dry etching, form the reflection grating that a plurality of vertical facets are formed.In order to reduce loss, increase reflectivity, can on the reflecting surface of etching moulding, plate the layer of metal reflection horizon usually.

1, output aberrationless position determines

For the periodicity grating, its grating equation is: n _EffD (sin θ _i+ sin θ _d)=m λ (1)

Here n _EffBe the effective refractive index of waveguide, d is the grating cycle, θ _iAnd θ _dBe respectively the incident angle and the angle of diffraction (as Fig. 1) of light, m is the grating diffration exponent number, and λ is an optical wavelength.

Specify an aberrationless point as output point, angle of diffraction of its correspondence and wavelength are respectively θ _D0And λ ₀So,, have:

n _effd(sinθ _i+sinθ _d0)＝mλ ₀ (2)

According to (1) and (2), we can predict that the wavelength of the light wave that may enter input waveguide is:

λ _p＝2mλ ₀(sinθ _i)/[p(sinθ _i+sinθ _d0)]，p＝1，2，... (3)

Here p is the diffraction of light exponent number that input waveguide is got back in coupling.

If the spectrum of wavelengths of incident comprises λ _pIn any one wavelength, then can be formed undesirable return loss by optical grating reflection and gather the input waveguide position.

The light distribution that each grating tooth produces separately has shape identical except that the intensity difference, and this intensity distributions is to have function S inc ²(x) shape, if input waveguide is placed the 0 value point place of this distribution, input waveguide will receive and approach 0 mirror field (in ideal conditions, equaling 0).

Here choose near λ ₀Wavelength as determining this Sinc ²(x) wavelength of shape envelope is got p=m, so:

λ _m＝2λ ₀(sinθ _i)/(sinθ _i+sinθ _d0) (4)

We choose near Sinc at this wavelength place ²(x) peaked first 0 value point of shape has:

n _effa(sinα)＝λ _m (5)

Here α is the effective width of the grating flank of tooth, and α=d (cos θ is arranged _i) (as Fig. 2).α is the angle of input direction IP and reflection direction PO, α=α _i+ α _d, α and incident angle θ _iWith reflection angle θ _D0Relation be:

α＝θ _i-θ _d0. (6)

If fixed input position and incident angle, can obtain the angle of diffraction of outgoing position according to (4) (5) (6):

θ _d0＝θ _i-arcsin(2tgθ _i/m) (7)

Satisfy (7) formula if the outgoing waveguide place aberrationless of appointment is put relative grating diffration angle, and each grating flank of tooth all reflexes to this direction to incident light, output waveguide just has been in the most approaching single flank of tooth diffraction Sinc like this ²(x) peaked first 0 value point of shape, return loss is reduced to greatest extent.

2, grating determines

The design of grating is very important here, and it makes the light incident at same input position different wave length, focuses on different outgoing positions point-blank.The aberration of requirement imaging in specifying output area is very little, and it is linear that chromatic dispersion simultaneously also keeps.The structure of etching diffraction grating wavelength division multiplexer of the present invention is based on the design of Rowland circle (promptly round A), and incidence point and eye point are on the Rowland circle, and grating is being on the circle of radius with 2 times of Rowland radius of circles, and grating and Rowland circle are tangent.The light of different wave length can well be imaged on output point in difference incident like this, and the out of focus aberration of imaging and coma are 0, and total aberration is very little.

As shown in Figure 2, iconal can be written as:

IP _i+k+P _i+kO-(IP _i+P _iO)＝kmλ ₀/n _eff (8)

Here I is an incidence point, and O is the output point on the perpendicular line.P _i, P _I+kIt is the mid point of i and i+k the reflection flank of tooth on the grating.What (8) formula was determined is a series of ellipses (overlapping with O as I, then is round), and the crossing point of the circle that is radius with 2 times of Rowland radius of circles among these ellipses (or circle) and Fig. 2 promptly is the mid point of the grating flank of tooth asked.

After obtaining mid point, the direction that the flank of tooth is set is for reflecting the direction of incident light to the aberrationless output point, and the position of whole grating has just been determined.

3, design example and effect assessment

Design example adopts following parameter: effective refractive index n _Eff=1.4502, diffraction exponent number m=40, Rowland encloses radius R=20000 μ m, incident angle θ _i=π/4 (45 degree), design wavelength lambda ₀=1.5525 μ m (193.1THz).So can obtain forming the wavelength X of return loss according to formula (4) and formula (7) _m=1.5934 μ m, θ ₀=0.7354 (42.1340 degree).

After obtaining grating institute geared surface according to (8) formula, we calculate output field with the Kirchhoff-Huygens diffraction equation and distribute: $E_{\mathrm{out}}(P^{\′},\mathrm{\λ})=\frac{1}{2}{\left(\frac{n_{\mathrm{eff}}}{\mathrm{\λ}}\right)}^{\frac{1}{2}}\mathrm{\η}\underset{\mathrm{grating}}{\∫}\frac{E_{\mathrm{inc}}\left(P\right)}{\sqrt{\left|{\mathrm{PP}}^{\′}\right|}}({\cos \mathrm{\α}}_i+\cos {\mathrm{\α}}_d)e^{-j\left(2\mathrm{\π}\right|{\mathrm{PP}}^{\′}|/\mathrm{\λ})}\mathrm{ds}---\left(9\right)$

Here P ' be output area more arbitrarily, P is the point of optical grating reflection face, η is the reflectivity of grating face.α _iAnd α _dBe incident angle and angle of diffraction, as shown in Figure 2 with respect to the optical grating reflection face.

Shown in Figure 3 is the low return loss etching diffraction grating wavelength division multiplexer that got by the last designing institute optical field distribution in output face when the incident of different wave length input light.As can be seen, just in time at the minimum point place of diffractional field distribution envelope, the luminous energy that enters into input waveguide like this is very little in the position of input waveguide, has realized the purpose of low return loss.Further.Our superposition integral obtains the spectral response of output channel. $I\left(\mathrm{\λ}\right)={|\underset{\mathrm{output}}{\∫}{E}_{\mathrm{out}}(P^{\′},\mathrm{\λ}){E^{*}}_{\mathrm{eigen}}(P^{\′},\mathrm{\λ})-\mathrm{dy}|}^2---\left(10\right)$

Here E _Eigen(P λ) is the eigenmode thing distribution of output waveguide.In order to compare, provide two contrast designs here with the normal etch diffraction grating wavelength division multiplexer that does not design through low return loss.These two gratings keep identical n with low return loss etched diffraction grating _Eff, m, R, θ _iValue, wherein first grating makes λ ₀=λ _m=1.5934 μ m reflection gathers input point, i.e. θ _D0=θ _i, incident light can be coupled fully and get back to the incident waveguide this moment; The output aberrationless point of second grating departs from 0 value point, θ d ₀The wavelength X that input waveguide is got back to in=0.7604 (43.5670 degree) and coupling _m=1.5724.

Shown in Figure 4 is the etching diffraction grating wavelength division multiplexer of low return loss design and the comparison that two kinds are not adopted the etching diffraction grating wavelength division multiplexer of low return loss design (gathering the design of incidence point and the design of the low return loss of common nothing) in input waveguide place optical field distribution.Gather the field strength distribution maximum that the designing institute of incidence point gets as can be seen, because nearly all incident light all enters input waveguide; It is lower slightly that the low return loss designing institute of common nothing gets the distribution of light intensity distribution, but can form big return loss; And low return loss designing institute gets light intensity and is significantly less than the above two, and return loss is able to effective reduction.

Shown in Figure 5 is that the etching diffraction grating wavelength division multiplexer of low return loss design adopts low return loss design (to gather the design of incidence point with two kinds; Design with the low return loss of common nothing) etching diffraction grating wavelength division multiplexer is in the comparison of input waveguide intermediate frequency spectrum response.Find out that from the peak of spectrum distribution the designing institute that to gather incidence point gets return loss near 0dB, the low return loss designing institute of common nothing gets return loss and is-3.7dB, can not satisfy low return loss requirement far away; And low return loss designing institute gets return loss and is-47.7dB to have reduced return loss to greatest extent.

Claims (2)

1. one kind low return loss etching diffraction grating wavelength division multiplexer comprises input waveguide (1), output waveguide (2), etched grating (3), free propagation region (4); It is characterized in that on the incidence point of input waveguide (1) and output waveguide (2) and the circle (A) that eye point is positioned at same radius, and etched grating (3) is positioned at 2 times of circles (A) radius is radius, and with circle (A) mutually on the circle of inscribe (B), the center of etched grating is on the point of contacts of two circles; Each reflection flank of tooth (7) of etched grating towards all determining by incident direction and exit direction, make incident light reflex to same output aberrationless point position (6).

2. low return loss etching diffraction grating wavelength division multiplexer according to claim 1 is characterized in that incidence point position (5) and the satisfied θ that concerns in output aberrationless point position (6) that said input waveguide is determined _D0=θ _i-arcsin (2tg θ _i/ m), make the minimum value place of incidence point position (5) at diffraction light distribution envelope; θ in the formula _D0Be the angle of diffraction of output aberrationless point position with respect to etched grating, θ _iBe the incident angle of incidence point position with respect to etched grating, m is the grating diffration exponent number.

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