CN1934476A - Planar waveguide reflective diffraction grating - Google Patents

Abstract

The invention relates to a planar waveguide reflective diffraction grating for use in an optical device, such as a wavelength division multiplexer, providing an increased bandwidth over conventional planar waveguide reflection diffraction gratings while eliminating the polarization dependent loss (PDL) typically associated therewith. Accordingly, a low order (<3), high a spect ratio (>10) grating is provided with a very short side wall (less than the wavelength of the optical signal) for use with incident angles of less than 15 DEG .

Description

Planar waveguide reflective diffraction grating

Technical field

The present invention relates to a kind of reflecting diffraction grating that is used for optical communication, relate more specifically to a kind of planar waveguide reflective diffraction grating device with Free Spectral Range of the polarization sensitivity that reduces and increase.

Background technology

In optical system, diffraction grating is an array meticulous, parallel on reflection or the transparent substrates, equidistant from distance groove (" groove "), and this groove causes diffraction and mutual interference effect, and it is along reflection in the discrete direction set or transmission electromagnetic energy, be called " level ", or " spectral order ".

Groove dimensions is similar to described wavelength in question with spacing.In optical field, the use of diffraction grating is the most general, and every millimeter has up to a hundred or thousands of groove.

Zero level is corresponding to direct transmission or direct reflection.Higher level causes incident beam and deviation in driction by how much (light) optics predictions.Utilize the normal angle of incident, the expression diffracted ray is provided by following equation with the angle θ of the deviation in driction of being predicted by geometrical optics, and wherein m is a spectral order, and λ is a wavelength, and d is the spacing between the adjacent trenches appropriate section:

$\mathrm{\&theta;}=\&PlusMinus;{\sin }^{-1}\left(\frac{\mathrm{m\&lambda;}}{d}\right)$

Because the angle of deviation of diffracted beam is relevant with wavelength, so diffraction grating is chromatic dispersion, and promptly it is its composition wavelength component with the incident beam apart, generation spectrum.

The groove number that depends on per unit distance on the spectral content of incident beam and the grating can be overlapping by the spectral order that diffraction grating produces.Spectral order is high more, overlapping big more to next even lower level.Diffraction grating is generally used in monochromator and other optical instruments.

By the cross sectional shape of control groove, it is possible concentrating on most of diffraction energies in the level that needs.This technology is called " glittering ".

At first, high resolution diffraction gratings is by groove.The structure of high-quality dividing machine is a big task.After a while, photoetching technique allows grating to produce from the holographic interference pattern.It is therefore not too bright that holographic grating has sinusoidal grooves, but because they cause much lower diffused light grade than blazed grating, be preferred in monochromator therefore.Reproduction technology allows high-quality duplicate to obtain from master grating, and this helps the reduction of grating cost.

Planar waveguide reflective diffraction grating comprises facet (facet) array of arranging with rule ordering.The performance of simple diffractive grating illustrates with reference to figure 1.Have a plurality of wavelength channel ₁, λ ₂, λ ₃Light beam 1 with specific incident angle θ _InEntering grating space is that Λ, the order of diffraction are the diffraction grating 2 of m.According to grating equation, light beam is to depend on the angle θ of wavelength and level then _OutAngular dispersion takes place:

mλ＝Λ(sinθ _in+sinθ _out) (1)

From grating equation (1), the condition that the order of diffraction forms depends on the wavelength X of incident light _NWhen considering the formation of spectrum, be necessary to know diffraction angle _NoutHow along with incident wavelength θ _InChange.Therefore, by with equation (1) to θ _NoutDifferential is supposed incident angle θ _InFixing, then derive following equation:

${\&PartialD;\mathrm{\&theta;}}_{\mathrm{Nout}}/\&PartialD;\mathrm{\&lambda;}=m/{\mathrm{\&Lambda;}\cos \mathrm{\&theta;}}_{\mathrm{Nout}}---\left(2\right)$

Micro component d θ _Nout/ d λ is the diffraction angle corresponding to the little variation of wavelength X _NoutVariation, this is known as the angular dispersion of diffraction grating.When level m increase, grating space Λ reduces, and diffraction angle _NoutDuring increase, angular dispersion increases.The linear dispersion of diffraction grating is the product of the effective focal length of this condition and system.

Because different wave length λ _NLight with different angles θ _NoutDiffraction, therefore every grade of m is stretched in the spectrum.Can be limited by grating space Λ by the progression that given diffraction grating produces, this is because θ _NoutCan not be above 90 °.Highest by Λ/provide.Therefore, coarse grating (having big Λ) produces many levels and meticulous grating can only produce one-level or two-stage.

The Free Spectral Range of diffraction grating (FSR) is defined as to bandwidth maximum in the deciding grade and level, and this grade is not overlapping with the same band in the adjacent level.Level m is important in determining Free Spectral Range, obtains continuous chromatic dispersion in this scope.For given input-grating-export structure, when grating is worked at the preferred order of diffraction m for optimal wavelength λ, other wavelength will adopt the path identical with other order of diffraction.When following equation is set up, the first overlapping generation of level:

mλ _m＝(m+1)λ _m+1 (3)

${\mathrm{\&lambda;}}_{m+1}=\frac{{\mathrm{m\&lambda;}}_m}{(m+1)}---\left(4\right)$

$\mathrm{\&Delta;\&lambda;}=\frac{\mathrm{\&lambda;m}}{m+1}---\left(5\right)$

Blazed grating is the grating that the groove of wherein diffraction grating is controlled to form the right-angle triangle with blazing angle w, as shown in Figure 1.The selection of blazing angle w particularly provides the chance of the total efficiency pattern of optimizing diffraction grating for setted wavelength.

For dense wave division multipurpose (DWDM), provide fabulous performance near infrared (IR) zone (1550nm) based on the device of planar waveguide diffraction.Particularly, the progress of echelle grating causes interfering the big phase differential between the path, and this grating is usually with high diffracting grade (40 grades to 80 grades), high incident angle (about 60 °) and big grating space work.Because size and the order of diffraction of grating facet are proportional, so recognized very early that this big phase differential is necessary for the reliable manufacturing based on the plane waveguiding device of diffraction.Thereby because the high diffracting grade (square journey 5) that needs, existing device is limited in work in the small wavelength scope.

In addition, for the device of in planar waveguide platform, making based on diffraction grating, the common problem that runs in the prior art is a Polarization Dependent Loss, and the field of a kind of polarization that it is caused by the existence with reflection facet F adjacent conductive metal S (reflectance coating) is repelled and caused.

Propagation has uncertain polarization state by the optical signalling of optical fiber, and it needs multiplexed (demultiplexing) device is polarization-insensitive basically, so that minimize Polarization Dependent Loss.In near the reflection grating of using the Littrow condition, and near the blazed grating the Littrow condition, two kinds of polarized lights are reflected well from reflection facet (F Fig. 1) in the same manner.Yet metalized sidewall facet S introduces boundary condition, prevents that polarization parallel is present in this near surface in the light (TM) on surface.In addition, compare with another kind of polarized light, a kind of polarized light is preferably absorbed by the metal on the sidewall S.Finally, the existence of side-wall metallic shows that the metal itself in the device act as Polarization Dependent Loss (PDL).

There are many method and apparatus that reduce the polarization sensitivity of diffraction grating.Chowdhury is in U.S. Pat 5,966,483 and US 6,097,863 in described by selecting to reduce polarization sensitivity with difference between first and second diffraction efficiencies that reduce wavelength in the transmission bandwidth.This scheme has the practicality that is restricted, because it requires the restriction of the selection aspect of blazing angle and blaze wavelength.

Sappey etc. are in U.S. Pat 6,400, have instructed polarization sensitivity to reduce by comprising by the reflective steps surface of planar separation (reflective step surface) and horizontal rise surface in 509.This scheme also has restricted practicality, and other surfaces do not require because it requires to have on some surfaces reflectance coating, causes needs to select to handle the additional manufacturing step of reflecting interface.

The size of the Free Spectral Range of grating and grating facet is proportional.Considered for a long time that the grating with little order of diffraction can not form reliably by photoengraving, because low level means usually less than the level that maybe can make photoetching resolution.Photoetching resolution and treatment step subsequently are stained and reduced the performance of grating basically.Therefore, the field of etched diffraction grating itself reaches the reasonably big order of diffraction because actual cause will limit, the general level 10 that surpasses.Having the realization that grade scope approaches 1 grade device has thought unpractiaca for a long time.

Defective by providing a kind of planar waveguide reflective diffraction grating to overcome prior art is provided, and wherein this grating has very little PDL owing at low relatively level work, and the bandwidth that increase is provided owing to very little sidewall length.

Summary of the invention

Therefore, the present invention relates to the reflection diffracting light gate device on a kind of planar waveguide platform that is used for demultiplexed optical signals, comprising:

Input port comprises the light beam of a plurality of wavelength channels that limited by mean wavelength with diffraction grating incident angle emission;

Reflecting diffraction grating is used for according to wavelength with multiple angles dispersive wave long channel, and reflecting diffraction grating has a plurality of reflecting walls that limited by facet length, and a plurality of sidewall that is limited by sidewall length; With

A plurality of output ports, the location is to catch wavelength channel;

Wherein the aspect ratio that limits of the facet length of being divided by sidewall length is greater than 3.

Another aspect of the present invention relates to the reflection diffracting light gate device on a kind of planar waveguide platform that is used for the optical channel that multiplexing or demultiplexing limits by mean wavelength, comprising:

Reflecting diffraction grating, it comprises a plurality of reflecting walls that limited by facet length, and a plurality of non-reflective side walls that is limited by sidewall length;

Input port is used for launching the light beam that comprises optical channel with diffraction grating place incident angle;

First output port is used for exporting of optical channel; With

Second output port is used for exporting another of optical channel;

Wherein selecting facet length and incident angle is 7 or the diffraction of even lower level to guarantee that grating provides absolute value.

Another feature of the present invention is provided for the reflection diffracting light gate device on the planar waveguide platform of demultiplexed optical signals, comprising:

Input port is used for comprising with diffraction grating incident angle emission the light beam of a plurality of wavelength channels that limited by mean wavelength;

Reflecting diffraction grating is used for according to wavelength with multiple angles dispersive wave long channel, and reflecting diffraction grating has a plurality of reflecting walls that limited by facet length, and a plurality of sidewall that is limited by sidewall length; With

A plurality of output ports, the location is to catch wavelength channel;

Wherein sidewall length is less than or equal to the twice of mean wavelength.

Description of drawings

Describe the present invention in detail below with reference to the appended accompanying drawing of representing the preferred embodiment of the present invention, wherein:

Fig. 1 shows the traditional reflective diffraction grating;

Fig. 2 shows according to planar waveguide reflective diffraction grating of the present invention; With

Fig. 3 shows the optical device that comprises according to planar waveguide reflective diffraction grating of the present invention.

Embodiment

Main concerned issue is to reflect the difference manufacturability with sidewall facet F and S in the design of planar waveguide diffraction grating.In addition, up to now, the major limitation of the manufacturability of facet is the photoetching resolution restriction.The resolving range of general photoetching process restriction is 0.5 μ m～1.0 μ m, and the minimum that therefore reaches the rational behavior of grating requires must be greater than this resolution for reflection facet size F, promptly is of a size of 2.5 μ m～5 μ m or large scale more.

In Fig. 1, by supposing input and output angle θ respectively _InAnd θ _NoutThe identical light path of simplifying.The mathematics manipulation of facet geometry is only simplified in this supposition.Therefore:

F ≈ Λ cos θ _InAnd (6)

Equation (1) is reduced to

mλ≈2Λsinθ _in (7)

Release in conjunction with equation 6 and 7

$F\&ap;\frac{\mathrm{m\&lambda;}}{{2\tan \mathrm{\&theta;}}_m}---\left(8\right)$

From Fig. 1:

$\frac{S}{F}\&ap;{\tan \mathrm{\&theta;}}_m---\left(9\right)$

In history, use 45 ° to 65 ° incident and output angle to cause grating facet aspect ratio F/S to be approximately 1 (seeing Fig. 1 and equation 9) inevitably.At wavelength 1550nm place, find from equation (6) that for reflection F and non-reflecting surface S it is to realize easily that the facet size of 10-17 μ m is used for DWDM in the prior art.This makes grating facet F to make, and is cost with big non-reflection facet (or sidewall) S that acts on Polarization Dependent Loss still.In the prior art, facet size changes also passes through to change order of diffraction m realization, promptly regulates the molecule of equation (8).

Communication network is by DWDM to CWDM and FTTH network development.Latter two network architectures has the channel of crossing over big wavelength coverage, and this scope is from～1250nm to～1630nm.These wide scopes are not suitable for the high diffracting grade device, and need be low to moderate 1 level usually.The professional of prior art does not recognize or utilizes equation (8).Operating angle θ low order of diffraction m and 45 ° to 65 ° _InAnd θ _OutThe place, the facet size F of the planar waveguide diffraction grating that obtains thus will be too little so that in fact can not make.Existing device based on planar waveguide diffraction comprises AWG and echelle grating.They all rely on high diffracting grade; AWG is because the guiding route cause need be at high level work, and the echelon grating technology uses high level to keep big facet size, its easier manufacturing.Therefore, in prior art has aspect addressing CWDM or the FTTH network structure in planar waveguide platform in restriction.

The present invention recognizes the importance of equation (8), particularly in fact may pass through the aspect ratio F/S of the angular dependence (-dance) increase grating facet of denominator.When angle of diffraction reduced, facet size was with tan θ _InLinear increasing.In addition, the present inventor recognizes that the increase of facet aspect ratio F/S produces Polarization Dependent Loss and improves and the bigger device of Free Spectral Range.

For example, in siliceous deposits silicon dioxide, 5 or the littler order of diffraction (producing minimum actual Free Spectral Range) for CWDM or FTTH network, at wavelength 1550nm place and the size of reflection facet F when surpassing 5.0 μ m, to need F/S to be increased to greater than 3, this can realize to about 25 ° by reducing angle of diffraction.Thereby the present invention comprises all planar waveguide diffraction grating designs, and wherein the ratio of reflection and non-reflection facet (or sidewall) is at least 3.

The amount of PDL depends on the length of aspect ratio F/S and non-reflection facet S very doughtily.The design of traditional echelon grating has～1 aspect ratio, and be subjected to the relevant PDL of sidewall very doughtily; Yet for the F/S that surpasses 3, non-reflection facet is very little basically to the contribution of PDL.By further increase F/S, might design and can make facet, the size S of its non-reflection grating facet is equal to or less than catoptrical wavelength, and for example S≤3000nm is preferred≤2500nm, more preferably≤and 2000nm, preferably last≤1550nm.Have the grating of metalized sidewall for these, the interaction length of light is very little so that no PDL (PDL-free) work of device becomes possibility.

Therefore, when we enter the wherein little state of tan (θ), realize that promptly aspect ratio is 1/3 or θ＜25 °, we can reduce sidewall correlativity PDL.

From the manufacturability viewpoint, if reflection facet F is big, then no matter facet itself is duplicated and faithfully the restriction of photoetching resolution.Little non-reflection facet S is unlikely duplicated faithfully, and sphering slightly, but the grating performance can not be affected.The professional of prior art recognizes that undoubtedly spacing is being controlled chromatic dispersion according to equation (1).Yet, very generally, make the spacing of grating equal vertical range between the reflection facet (sidewall S among Fig. 1).Utilize this idea, the distortion of sidewall S can equal the distortion of grating space.This is the notion of a mistake, and in fact grating space is provided by equation (6).With counter-intuitive ground, spacing is along with F rather than S increase.The present inventor recognizes this fact, and can increase aspect ratio, promptly reduces S/F, shown in equation (9), and does not influence the risk of grating space.In fact, the fidelity of grating replication is not by photoetching but the accuracy limit of masked unique characteristics.The little several magnitude of this restriction ratio photoetching resolution (hundred times).

In conjunction with equation (8) and (9), we find:

$S\&ap;\frac{\mathrm{m\&lambda;}}{2}---\left(10\right)$

Thereby, by select the little order of diffraction (m=3,2, or 1, if necessary), can almost eliminate PDL, because side wall dimensions S is less than wavelength.

In a preferred embodiment, as shown in Figure 3, recessed reflecting diffraction grating 10 forms in the edge that is arranged at the planar waveguide 11 on the chip 12.Input port is limited by the end of waveguide 13, and its edge from chip 12 extends to planar waveguide 11, is used for transmission and comprises a plurality of wavelength channel (λ ₁, λ ₂, λ ₃) input wavelength-division multiplex (WDM) signal.Limit with reference to figure 2 as top, diffraction grating 10 has the aspect ratio (F/S) greater than 5, and sidewall length S is less than or equal to wavelength channel (λ ₁, λ ₂, λ ₃) mean wavelength.Input waveguide 13 is positioned to guarantee incident angle θ _InLess than 30 °, grating space Λ is selected to guarantee that grating 10 provides 5 or the diffraction of littler level.Diffraction grating 10 is separated into the composition wavelength with input signal, and each wavelength channel is gathered independent output port with the form of output waveguide 15, its end is provided with along the focal line 16 of the grating 10 that is limited by rowland (Rowland) circle, is used to transmit the edge of getting back to chip 12.Shown device also can be used for multiplexing several wavelength channel, imports described waveguide 15, becomes the single output signal that is transferred to chip 12 edges via input waveguide 13.Input and output port is represented the position in the slab guide 11, is launched or catches at this position light; Yet port can or be blocked simply with other transmission apparatus optical coupled.

Operate specifically being exemplified as of above-mentioned optical device:

θ _in＝5° 5° 5° 6°

m＝1 2 3 2

λ _avg＝1550nm 1550nm 1550nm 1550nm

Λ＝8892nm 17784nm 26676nm 14828nm

F＝8858nm 17716nm 26574nm 14747nm

S＝775nm 1550nm 2325nm 1550nm

F/S＝11.4 11.4 11.4 9.5

Claims (20)

1. the reflection diffracting light gate device on the planar waveguide platform that is used for demultiplexed optical signals comprises:

Input port is used for comprising with diffraction grating incident angle emission the light beam of a plurality of wavelength channels that limited by mean wavelength;

Reflecting diffraction grating is used for disperseing described wavelength channel according to wavelength with multiple angles, and described reflecting diffraction grating has a plurality of reflecting wall and a plurality of sidewalls that limited by sidewall length that limited by facet length; And

A plurality of output ports are positioned to catch described wavelength channel;

Wherein aspect ratio is greater than 3, and described aspect ratio equals described facet length divided by described sidewall length.

2. device according to claim 1, wherein said aspect ratio is greater than 5.

3. device according to claim 1, wherein said aspect ratio is greater than 10.

4. device according to claim 3, wherein said diffraction grating incident angle is less than 6 °.

5. device according to claim 3, wherein said sidewall length is less than or equal to the twice of described mean wavelength.

6. device according to claim 1, wherein said sidewall length is less than or equal to described mean wavelength.

7. device according to claim 1, wherein said incident angle is less than 30 °.

8. device according to claim 1, wherein said incident angle is less than 15 °.

9. the reflection diffracting light gate device on the planar waveguide platform that is used for the optical channel that multiplexing or demultiplexing limits by mean wavelength comprises:

Reflecting diffraction grating, it comprises:

A plurality of reflecting walls that limit by facet length, and

A plurality of non-reflective side walls that limit by sidewall length;

Input port is used for launching the light beam that comprises described optical channel with described diffraction grating place incident angle;

First output port is used for exporting of described optical channel; With

Second output port is used for exporting another of described optical channel;

Wherein selecting described facet length and described incident angle is 7 or the diffraction of even lower level to guarantee that described grating provides absolute value.

10. device according to claim 9, wherein said level is 5 or lower.

11. device according to claim 9, its middle rank are 3 or lower.

12. device according to claim 11, wherein said diffraction grating incident angle is less than 6 °.

13. device according to claim 12, wherein said sidewall length is less than or equal to described mean wavelength.

14. device according to claim 9, wherein said incident angle is less than 45 °.

15. device according to claim 9, wherein said incident angle is less than 30 °.

16. device according to claim 9, wherein said incident angle is less than 15 °.

17. device according to claim 9, wherein said sidewall length is less than or equal to the twice of described mean wavelength.

18. device according to claim 9, wherein said sidewall length is less than or equal to described mean wavelength.

19. the reflection diffracting light gate device on the planar waveguide platform that is used for demultiplexed optical signals comprises:

Input port is used for comprising with diffraction grating incident angle emission the light beam of a plurality of wavelength channels that limited by mean wavelength;

Reflecting diffraction grating is used for disperseing described wavelength channel according to wavelength with multiple angles, and described reflecting diffraction grating has a plurality of reflecting wall and a plurality of sidewalls that limited by sidewall length that limited by facet length; And

A plurality of output ports are positioned to catch described wavelength channel;

Wherein said sidewall length is less than or equal to the twice of described mean wavelength.

20. device according to claim 19, wherein said sidewall length is less than or equal to the twice of described mean wavelength.

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