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

Abstract

The invention discloses a low return loss etching diffraction grating wavelength division multiplexer. It includes input waveguide, output waveguide, etched grating, free propagation area. The input point and output point of the input waveguide and output waveguide are located on a circle with the same radius, and the etched grating is located on another circle with a radius twice the radius of this circle and inscribed with this circle. The center of the etched grating is at two On the tangent point of the circle. The orientation of each reflective tooth surface of the etched grating is determined by the incident direction and the outgoing direction, so that the incident light is reflected to the same output aberration-free point position. The input position and the output aberration-free point position satisfy a precise relationship, which can ensure that the position where the input waveguide is located receives the smallest optical return loss. The present invention can be better applied in the wavelength division multiplexing system, while it maximizes the performance of the return loss, compared with the common method of reducing the return loss (such as using an optical isolator), there is no additional geometric The structure and manufacturing process are not changed, the realization is simple, and the cost does not increase.

Description

Low return loss etched diffraction grating wavelength division multiplexer

Technical field

The invention relates to the field of optical communication wavelength division multiplexing, in particular to a low return loss etching diffraction grating wavelength division multiplexer.

Background technique

Wavelength Division Multiplexing (WDM) technology can multiplex tens or even hundreds of wavelengths in a single fiber to meet the increasing demand for bandwidth, and is one of the effective methods to improve communication capabilities. Wavelength division multiplexing technology has been improved so that it can transmit hundreds of wavelengths of laser light, and the interval between each wavelength is 1.6, 0.8 or 0.4nm, or even 0.2nm. This technology is called Dense Wavelength Division Multiplexing (DWDM). WDM has been widely used in long-distance transmission, and has also begun to enter short-distance optical communication networks.

The wavelength division multiplexer is the most critical device in the wavelength division multiplexing optical fiber communication system. The existing realization technologies mainly include thin-film filter, fiber grating, integrated planar waveguide and so on. Among them, the integrated planar waveguide wavelength division multiplexer can realize dense wavelength division multiplexing of more than 40 channels on a very small chip, and it can be mass-produced by using semiconductor technology, which has a potential cost advantage. The most typical integrated planar waveguide wavelength division multiplexer is arrayed waveguide grating (AWG), its technology is relatively mature, and 40-channel products have been commercialized. The other is an integrated planar waveguide wavelength division multiplexer based on etched diffraction grating (EDG), which has a smaller size, enables single-sided packaging, and is easy to achieve higher channel count wavelength division multiplexing. Although EDG requires a higher manufacturing process than AWG, it is still a very promising integrated planar waveguide wavelength division multiplexer.

Return loss is an important indicator of an optical device. It refers to the ratio of the light energy reflected back to the incident path in the optical path to the total incident light energy, expressed in dB. The return loss weakens the performance of the laser, and the multiple refraction and reflection of light in the optical path coherently affects the quality of the optical signal, which increases the noise and the bit error rate. Generally, the return loss of optical devices comes from end surface reflection (Fresnel reflection) and material backscattering (Rayleigh backscattering). The integrated planar waveguide wavelength division multiplexer based on etched diffraction grating is a reflective grating type device, so the source of return loss is grating reflection in addition to end surface reflection and material scattering. In the etched diffraction grating wavelength division multiplexer, the light of most wavelengths is coupled into each output waveguide, and the input waveguide may only receive light of a specific wavelength range. If the input light does not contain these wavelengths, reflections from the grating will not cause return loss. However, due to environmental influences such as temperature, the wavelength of the incident light will drift. If the wavelength drift reaches the wavelength that the input waveguide can receive, return loss will occur. In addition, the background noise of the EDFA used for optical signal amplification in the optical communication system has a wide frequency spectrum, and the noise light returns to the incident waveguide, which will cause return loss.

The general way to reduce the return loss is to use an interface with an inclined angle, but this method can only reduce the return loss caused by the interface, and it is invalid for the return loss caused by other factors such as grating reflection. Using an optical isolator in the optical path can reduce the overall return loss, but the use of an optical isolator increases the cost and complexity, and adds additional insertion loss. Due to the close proximity of the input and output positions of the etched diffraction grating wavelength division multiplexer, the source of the return loss of the device is mainly the reflection of the grating. The invention proposes a design method for manufacturing a low return loss etching diffraction grating wavelength division multiplexer. While maximizing return loss performance, this method does not add any geometric structure and manufacturing process compared with ordinary methods for reducing return loss (such as using optical isolators), and is simple to implement and low in cost. Increase.

Contents of invention

The purpose of the present invention is to provide a low return loss etched diffraction grating wavelength division multiplexer to ensure the minimum optical return loss received by the input waveguide.

The technical scheme that the present invention adopts is as follows:

It includes input waveguide, output waveguide, etched grating, free propagation area. The incident point and exit point of the input waveguide and output waveguide are located on a circle with the same radius, and the etched grating is located on another circle that is twice the radius of this circle and inscribed with this circle. The center of the etched grating is on the two circles on the cutting point. The orientation of each reflective tooth surface of the etched grating is determined by the incident direction and the outgoing direction, so that the incident light is reflected to the same output aberration-free point position.

The position of the incident point determined by the input waveguide and the position of the output aberration-free point satisfy the relationship θ _d0 =θ _i -arcsin(2tgθ _i /m), so that the position of the incident point is at the minimum value of the diffracted light distribution envelope; where θ _d0 is the diffraction angle of the output aberration-free point position relative to the etched grating, _θi is the incident angle of the incident point position relative to the etched grating, and m is the diffraction order of the grating.

The present invention adopts mature semiconductor deposition and etching processes to deposit a single-mode slab waveguide on a semiconductor material substrate (for example, a three-layer silicon dioxide optical waveguide is produced by plasma-enhanced chemical vapor deposition on a silicon substrate), and the light can Single-mode transmission in the core layer of the waveguide, input and output waveguides and diffraction gratings are obtained by dry etching (for example, silicon dioxide is etched by inductively coupled plasma).

The advantages of the present invention are: while maximizing the return loss performance, compared with the common method of reducing return loss (such as using an optical isolator), there is no additional geometric structure and no change in the manufacturing process, realizing Simple, no cost increase.

Description of drawings

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

Figure 2 is an enlarged view of the design principle of the grating tooth surface of the low return loss etching diffraction grating wavelength division multiplexer;

Figure 3 is the light field distribution on the output surface of the low return loss etched diffraction grating wavelength division multiplexer when input light of different wavelengths is incident;

Figure 4 shows the etched diffraction grating wavelength division multiplexer with low return loss design and the etched diffraction grating wavelength division multiplexer without low return loss design (the design focused on the incident point, and the ordinary design without low return loss) Comparison of the optical field distribution of the filter at the input waveguide;

Figure 5 shows the etched diffraction grating wavelength division multiplexer with low return loss design and the etched diffraction grating wavelength division multiplexer without low return loss design (the design focused on the incident point, and the ordinary design without low return loss) A comparison of the spectral response of the filter in the input waveguide.

Detailed ways

The low return loss etched diffraction grating wavelength division multiplexer shown in FIG. 1 includes an input waveguide 1 , an output waveguide 2 , an etched grating 3 , and a free propagation area 4 . The incident point and the exit point of the input waveguide 1 and the output waveguide 2 are located on a circle A with the same radius, and the etched grating 3 is located on a circle B whose radius is twice the radius of the circle A and is inscribed with the circle A. The etched grating The center is at the point of tangency of the two circles. As shown in FIG. 2 , the orientation of each reflective tooth surface 7 of the etched grating is determined by the incident direction and the outgoing direction, so that the incident light is reflected to the same output aberration-free point position 6 .

The incident point position 5 determined by the input waveguide and the output aberration-free point position 6 satisfy the following relationship: θ _d0 = θ _i -arcsin(2tgθ _i /m), so that the incident point position 5 is at the minimum value of the diffracted light distribution envelope; Among them, θ _d0 is the diffraction angle of the output aberration-free point position relative to the etched grating, θ _i is the incident angle of the incident point position relative to the etched grating, and m is the diffraction order of the grating.

The low return loss etching diffraction grating wavelength division multiplexing device of the present invention uses semiconductor technology to coat thin films on silicon wafers or other base materials with PECVD (plasma enhanced chemical vapor deposition) or FHD (flame water oxygen hair) and other processes to form slab waveguide. The input and output waveguides and diffraction gratings are dry-etched by RIE (reactive ion etching) or ICP (inductively coupled plasma etching) to form reflective gratings composed of multiple vertical facets. In order to reduce loss and increase reflectivity, a metal reflective layer can usually be plated on the etched reflective surface.

1. Determination of output aberration-free position

For periodic gratings, the grating equation is: n _eff d(sinθ _i +sinθ _d )=mλ (1)

Here n _eff is the effective refractive index of the waveguide, d is the grating period, θ _i and θ _d are the incident angle and diffraction angle of light respectively (as shown in Figure 1), m is the diffraction order of the grating, and λ is the wavelength of light.

Designate an aberration-free point as the output point, and its corresponding diffraction angle and wavelength are θ _d0 and λ ₀ , then, there are:

n _eff d(sinθ _i +sinθ _d0 )＝mλ ₀ (2)

From (1) and (2), we can predict that the wavelengths of the light waves that may enter the input waveguide are:

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

Here p is the diffraction order of light coupled back into the input waveguide.

If the incident wavelength spectrum includes any one of the wavelengths in _λp , it will be reflected by the grating and concentrated to the input waveguide position, forming an undesired return loss.

The light intensity distribution produced by each grating tooth alone has the same shape except for the different intensity, and this intensity distribution has the shape of function Sinc ² (x), if the input waveguide is placed at the 0 value point of this distribution, the input waveguide will be will receive a reflected field close to zero (in ideal cases, equal to zero).

Here, the wavelength close to λ ₀ is selected as the wavelength for determining the envelope of this Sinc ² (x), and p=m is taken, so:

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

At this wavelength, we select the first 0-value point closest to the maximum value of the Sinc ² (x) shape, which is:

n _eff a(sinα)=λ _m (5)

Here α is the effective width of the grating tooth surface, and there is α=d(cosθ _i ) (as shown in Figure 2). α is the angle between the input direction IP and the reflection direction PO, α=α _i +α _d , the relationship between α and the incident angle θ _i and the reflection angle θ _d0 is:

α＝ _θi - _θd0 . (6)

If the input position and incident angle are fixed, the diffraction angle of the output position can be obtained according to (4)(5)(6):

θ _d0 ＝θ _i -arcsin(2tgθ _i /m) (7)

If the diffraction angle of the aberration-free point where the specified outgoing waveguide is located relative to the grating satisfies formula (7), and each grating tooth surface reflects the incident light to this direction, then the output waveguide is in the closest single tooth surface diffraction Sinc ² ( x) The first 0 value point of the maximum value of the shape, the return loss is minimized.

2. Determination of grating

The design of the grating is very important here. It makes the incident light of different wavelengths at the same input position focus on different output positions on a straight line. It is required that the imaging aberrations in the specified output range be small, while the dispersion is also linear. The structure of the etching diffraction grating wavelength division multiplexer of the present invention is based on the design of the Rowland circle (i.e. circle A), the incident point and the exit point are on the Rowland circle, and the grating is on a circle with a radius of 2 times the Rowland circle radius, and the grating Tangent to the Rowland circle. In this way, light of different wavelengths incident at different points can be well imaged at the output point, the defocus aberration and coma aberration of the image are zero, and the total aberration is very small.

As shown in Figure 2, the optical path function can be written as:

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

Here I is the incoming point and O is the outgoing point on the vertical line. P _i , P _i+k are the midpoints of the i-th and i+k-th reflective tooth surfaces on the grating. What formula (8) determines is a series of ellipses (if I and O coincide, then it is a circle), the point where these ellipses (or circles) intersect with the circle whose radius is twice the Rowland circle radius in Figure 2 is the desired point Midpoint of the grating tooth surface.

After obtaining the midpoint, set the direction of the tooth surface as the direction that can reflect the incident light to the output point without aberration, and the position of the entire grating is determined.

3. Design examples and effect evaluation

The design example adopts the following parameters: effective refractive index n _eff =1.4502, diffraction order m=40, Rowland circle radius R=20000 μm, incident angle θ _i =π/4 (45 degrees), design wavelength λ ₀ =1.5525 μm (193.1 THz). Therefore, according to formula (4) and formula (7), it can be obtained that the return loss wavelength λ _m =1.5934 μm, θ ₀ =0.7354 (42.1340 degrees).

After obtaining all the tooth surfaces of the grating according to (8), we use the Kirchhoff-Huygens diffraction equation to calculate the output field distribution: ${\mathrm{E.}}_{\mathrm{out}}(P^{\′},\mathrm{\λ})=\frac{1}{2}{\left(\frac{{\mathrm{no}}_{\mathrm{eff}}}{\mathrm{\λ}}\right)}^{\frac{1}{2}}\mathrm{\η}\underset{\mathrm{grating}}{\∫}\frac{{\mathrm{E.}}_{\mathrm{inc}}\left(P\right)}{\sqrt{\left|{\mathrm{PP}}^{\′}\right|}}({\cos \mathrm{\α}}_i+\cos {\mathrm{\α}}_d)e^{-j\left(2\mathrm{\π}\right|{\mathrm{PP}}^{\′}|/\mathrm{\λ})}\mathrm{ds}---\left(9\right)$

Here P' is an arbitrary point in the output area, P is a point on the reflective surface of the grating, and η is the reflectivity of the grating surface. α _i and α _d are the incident angle and diffraction angle relative to the reflective surface of the grating, as shown in Figure 2.

Figure 3 shows the light field distribution on the output surface of the low return loss etched diffraction grating wavelength division multiplexer designed above when input light of different wavelengths is incident. It can be seen that the position of the input waveguide is just at the lowest point of the envelope of the diffraction field distribution, so that the light energy entering the input waveguide is very small, and the purpose of low return loss is achieved. further. We superimpose the integrals to get the spectral response of the output channel. $I\left(\mathrm{\λ}\right)={|\underset{\mathrm{output}}{\∫}{\mathrm{E.}}_{\mathrm{out}}(P^{\′},\mathrm{\λ}){{\mathrm{E.}}^{*}}_{\mathrm{eigen}}(P^{\′},\mathrm{\λ})-\mathrm{dy}|}^2---\left(10\right)$

Here _Eeigen (P,λ) is the eigenmode distribution of the output waveguide. In order to compare with the ordinary etched diffraction grating wavelength division multiplexer without low return loss design, two comparative designs are given here. These two gratings maintain the same values of n _eff , m, R, θ _i as the low return loss etched diffraction grating, where the first grating makes λ ₀ = λ _m = 1.5934 μm reflections focused to the input point, i.e. θ _d0 = θ _i , the incident light can be fully coupled back to the incident waveguide at this time; the output of the second grating has no aberration and deviates from the zero value point, θd ₀ =0.7604 (43.5670 degrees) and the wavelength λ _m =1.5724 coupled back to the input waveguide.

Figure 4 shows the etched diffraction grating wavelength division multiplexer with low return loss design and the etched diffraction grating WDM without low return loss design (the design focused on the incident point, and the ordinary design without low return loss). Comparison of the optical field distribution at the input waveguide of the multiplexer. It can be seen that the field intensity distribution obtained by the design concentrated at the incident point is the largest, because almost all the incident light enters the input waveguide; the light field intensity distribution obtained by the ordinary design without low return loss is slightly lower, but still forms a large return loss; The light intensity obtained by the low return loss design is much lower than the former two, and the return loss can be effectively reduced.

Figure 5 shows the etched diffraction grating wavelength division multiplexer with low return loss design and the etched diffraction grating WDM without low return loss design (the design focused on the incident point; and the ordinary design without low return loss). Comparison of spectral responses of multiplexers in input waveguides. It can be seen from the highest point of the spectrum distribution that the return loss obtained from the design of the out-gathering point to the incident point is close to 0dB, and the return loss obtained by the ordinary design without low return loss is -3.7dB, which is far from meeting the low return loss requirement; while the low return loss design The designed return loss is -47.7dB, which minimizes the return loss.

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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