Transmission-type diffraction grating and related apparatus

Abstract

The application provides a polarization-independent transmission diffraction grating and related equipment. The transmission diffraction grating comprises a transparent substrate and a grating structure arranged in a stack, the grating structure comprises at least two grating layers, the bottommost grating layer of the grating structure on the transparent substrate is made of a high refractive index material, each grating layer except the bottommost grating layer is made of a high refractive index material or a low refractive index material, the materials of the two adjacent grating layers are different, the refractive index range of the low refractive index material is [1.30, 1.80], the refractive index range of the high refractive index material is [1.90, 2.50], the thickness range of the grating layer made of the low refractive index material is [100, 400] nm, and the total thickness range of the grating structure is [1000, 1450] nm. The transmission grating has high diffraction efficiency and polarization independence in a wide incident angle range near the Littrow angle.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical communication technology, in particular to a transmission type diffraction grating and related equipment. BACKGROUND

[0002] All Optical Network (AON) is a network in which signals are transmitted, exchanged and amplified in the form of optical signals without electronic processing, and only converted between electrical and optical signals when entering or leaving the network. AON is a direct fiber communication network based on optical fiber as the light propagation medium, which is not bound by the slow response of electronic devices in traditional networks, reduces network latency, and reduces system power consumption. AON based on Dense Wavelength Division Multiplexing (DWDM) technology can realize high-speed and high-capacity information transmission and processing, and is the trend of current communication technology development.

[0003] AON uses optical switching nodes to replace the electrical nodes of traditional networks. In order to meet the dynamic needs of network traffic, the optical switching node needs to have the ability to allocate resources on demand, that is, to dynamically configure uplink and downlink services, to support the flexibility of AON. At the same time, due to the increase of network services, the optical switching node needs to have multi-dimensional uplink and downlink ports to fully utilize the network capacity of wavelength division multiplexing technology and perform multi-dimensional service scheduling. Therefore, the optical switching node is composed of a Reconfigurable Optical Add-Drop Multiplexer (ROADM). ROADM is a device that adds, blocks, passes through, or redirects different wavelength optical signals in a fiber communication network. Through remote reconfiguration, it can dynamically configure uplink or downlink service wavelengths as needed to achieve flexible scheduling of services. Wavelength Selective Switch (WSS) is a key module of a multi-dimensional, uplink and downlink flexible ROADM. Wavelength Selective Switch can unobstructedly schedule any input port, any wavelength of optical signal to any output port after demultiplexing.

[0004] Diffractive gratings are commonly used diffractive optical devices in WSSs to realize wavelength division multiplexing. The diffractive gratings in WSSs usually adopt polarization-independent diffractive gratings with very high diffraction efficiency (usually required to be greater than 90%) to reduce optical loss in WSSs. However, polarization-independent gratings often have very deep grating structures, which are difficult to manufacture and are not conducive to the miniaturization of diffractive gratings. In addition, the diffraction efficiency and polarization response characteristics of the diffractive grating are sensitive to the incident angle. If the incident angle deviates from the Littrow angle, the diffraction efficiency will decrease. The Littrow angle is the incident angle that is the same as the diffraction angle. Therefore, the design of a diffractive grating with high diffraction efficiency, polarization independence and wide incident angle range is a technical problem at present. SUMMARY

[0005] Embodiments of the present application provide a polarization-independent transmissive diffractive grating with high diffraction efficiency in a wide incident angle range and a wavelength selective switch with the transmissive diffractive grating.

[0006] In a first aspect, the present application provides a polarization-independent transmissive diffractive grating, comprising a transparent substrate and a grating structure stacked together, wherein the grating structure comprises at least two grating layers, the bottommost grating layer of the grating structure on the transparent substrate is made of a high refractive index material, each grating layer except the bottommost grating layer is made of a high refractive index material or a low refractive index material, the materials of the two adjacent grating layers are different, the refractive index range of the low refractive index material is [1.30, 1.80], the refractive index range of the high refractive index material is [1.90, 2.50], the thickness range of the grating layer made of the low refractive index material is [100, 400] nm, the total thickness range of the grating structure is [1000, 1450] nm, the duty cycle range of the grating structure is [50%, 60%], and the line density of the grating structure is not less than 965 lines / mm.

[0007] The materials of the two adjacent grating layers are different, including two cases: the first case is that the materials of the two adjacent grating layers are different but have the same refractive index characteristics; the second case is that the materials of the two adjacent grating layers are different and have different refractive index characteristics. It can be understood that the characteristics of the materials of the two adjacent grating layers can both be low refractive index materials, both be high refractive index materials, or one be a low refractive index material and the other be a high refractive index material. However, in any of the above cases, the two adjacent grating layers are made of different materials. In other words, the refractive index of the two adjacent grating layers is different.

[0008] The surface of the diffraction grating is provided with a plurality of grooves to form a plurality of grating ridges on the diffraction grating. The plurality of grooves are periodic structures of sub-wavelength magnitude. The grating line density refers to the number of grooves (or slits or notches) per millimeter of the grating, and the duty cycle refers to the ratio of the width of the grating ridge to the grating period in one grating period. The grating period, also known as grating constant, is the distance between two grooves of the grating, and the grating period is the inverse of the grating line density.

[0009] The transmission type diffraction grating provided by the first aspect has a total thickness of the grating structure in the range of [1000, 1450] nm, and the thickness of the grating layer made of the low refractive index material is in the range of [100, 400] nm, that is, the total thickness of the sum of all grating layers made of the low refractive index material is in the range of [100, 400] nm, the grating layer closest to the bottom layer of the transparent substrate is made of the high refractive index material, and is applied to the C waveband [1524, 1575] nm of optical communication. For the incident light beam of TE and TM polarization modes, and the incident angle of the incident light beam is in the range of [θ-5.5°, θ+5.5°], wherein θ is the Littrow angle, the lowest diffraction efficiency of the transmission type diffraction grating-1 order is greater than 92%, the highest polarization loss is less than 0.2 dB, that is, the diffraction efficiency of the incident light beam of TE and TM polarization modes is less than 0.2 dB. In addition, compared with the total thickness of the conventional grating structure being greater than 1700 nm, the total thickness of the grating structure of the present application is in the range of [1000, 1450] nm, which effectively reduces the thickness of the transmission type diffraction grating in the stacking direction of the grating structure and the transparent substrate layer and the groove depth on the grating structure, which is beneficial to the miniaturization of the transmission type diffraction grating and reduces the difficulty of manufacturing the transmission type diffraction grating.

[0010] In a first possible implementation manner of the first aspect, the grating structure comprises a first grating layer and a second grating layer, the second grating layer is arranged between the first grating layer and the transmission substrate, the thickness of the first grating layer ranges from 240 nm to 330 nm, and the thickness of the second grating layer ranges from 1000 nm to 1120 nm, so that the lowest diffraction efficiency of the grating-1 order is greater than 96% and the highest polarization loss is less than 0.1 dB when the incident angle of the incident light beam in the optical communication C band [1524, 1575] nm ranges from θ-5.5° to θ+5.5°. Compared with a conventional double-layer grating structure (generally with a thickness ranging from 1700 nm), the grating structure has a smaller thickness and reduces the etching difficulty of the grating structure. Since each grating layer has a thickness tolerance, the cumulative tolerance is greater when the number of layers of the grating structure is greater. Compared with a conventional grating structure with more than three layers, the grating structure has only two layers, which reduces the cumulative tolerance between the layers and is beneficial to improve the diffraction efficiency and light transmission quality of the transmission diffraction grating.

[0011] In a second possible implementation manner of the first aspect, the high-refractive-index material comprises one of TiO2, Nb2O5 and Ta2O5, and the low-refractive-index material comprises one of SiO2, Al2O3, CaF2, LiF, MgF2, NaF, SrF2 and cryolite.

[0012] In a third possible implementation manner of the first aspect, the grating structure comprises a first grating layer, a second grating layer and a third grating layer arranged in sequence, the third grating layer is arranged between the transparent substrate and the second grating layer, the second grating layer is made of the low-refractive-index material, and the first grating layer is made of the high-refractive-index material. In this way, the grating structure comprises only three grating layers, which reduces the tolerance between the layers of the grating structure and further improves the diffraction efficiency of the transmission diffraction grating.

[0013] In a fourth possible implementation manner of the first aspect, the thickness of the first grating layer ranges from 20 nm to 45 nm, the thickness of the second grating layer ranges from 150 nm to 230 nm, and the thickness of the third grating layer ranges from 1030 nm to 1090 nm, so that the lowest diffraction efficiency of the grating-1 order is greater than 96% and the highest polarization loss is less than 0.1 dB when the incident angle of the incident light beam in the optical communication C band [1524, 1575] nm ranges from θ-5.5° to θ+5.5°.

[0014] In a fifth possible implementation manner of the first aspect or the first to fourth possible implementation manners of the first aspect, the grating structure comprises a first grating layer, a second grating layer and a third grating layer arranged in sequence, the third grating layer is arranged between the transparent substrate and the second grating layer, the second grating layer is made of the high-refractive-index material, the first grating layer is made of the low-refractive-index material, and the third grating layer is made of a material different from that of the second grating layer. In this way, the grating structure is composed of only three grating layers, the tolerance of the grating structure is reduced, and the diffraction efficiency of the transmission diffraction grating is further improved.

[0015] In a sixth possible implementation manner of the first aspect or the first to fifth possible implementation manners of the first aspect, the thickness of the first grating layer ranges from 200 nm to 350 nm, the thickness of the second grating layer ranges from 420 nm to 620 nm, and the thickness of the third grating layer ranges from 485 nm to 720 nm. In this way, when the incident angle of the incident light beam in the optical communication C band [1524, 1575] nm ranges from [θ-5.5°, θ+5.5°], the lowest diffraction efficiency of the grating-1 order is greater than 96%, and the maximum polarization loss is less than 0.1 dB.

[0016] In a seventh possible implementation manner of the first aspect or the first to sixth possible implementation manners of the first aspect, the high-refractive-index material comprises one of TiO2, Nb2O5, Ta2O5 and Si3N4, and the low-refractive-index material comprises one of SiO2, Al2O3, CaF2, LiF, MgF2, NaF, SrF2 and cryolite.

[0017] In an eighth possible implementation manner of the first aspect or the first to seventh possible implementation manners of the first aspect, the transparent substrate comprises fused quartz.

[0018] In a second aspect, the implementation manners of the present application further provide a wavelength selective switch, comprising an optical input port, an optical output port, an optical deflection component and any one of the transmission diffraction graters according to the first aspect. The optical input port is configured to input an incident light beam, the transmission diffraction grating is configured to split or combine the incident light beam, and the optical deflection component is configured to selectively deflect the light beam split or combined by the transmission diffraction grating to switch to a corresponding optical output port for output.

[0019] The incident light beam includes two or more wavelength signals, and the transmission diffraction grating is configured to spatially separate the incident light beam to form two or more sub-beams of different wavelengths. Alternatively, the incident light beam is multiple, and the multiple incident light beams are input through the optical input port and incident on the transmission diffraction grating, and the transmission diffraction grating combines the multiple incident light beams to form a processing light beam.

[0020] The wavelength selective switch provided in the second aspect has a transmission diffraction grating with a diffraction efficiency greater than 92% and a maximum polarization loss less than 0.2 dB, thereby greatly improving the light transmission efficiency and quality of the wavelength selective switch.

[0021] In a first possible implementation manner of the second aspect, the optical input port and the optical output port each include a fiber port and a collimating mirror corresponding to the fiber port. An incident light beam input through the fiber port of the optical input port is collimated by the collimating mirror of the optical input port and then transmitted to the transmission diffraction grating. The processing light beam deflected by the optical deflection component reaches the collimating mirror of the corresponding optical output port and is then output through the fiber port of the optical output port. In this way, the light transmission quality and efficiency of the wavelength selective switch are further improved.

[0022] In a second possible implementation manner of the second aspect, the wavelength selective switch includes a polarization optical component. An incident light beam input from the optical input port is adjusted in polarization state by the polarization optical component and then incident on the transmission diffraction grating, thereby improving the light transmission quality.

[0023] In a third possible implementation manner of the second aspect, the wavelength selective switch includes a beam shaping component. An incident light beam adjusted in polarization state by the polarization optical component is incident on the beam shaping component for shaping and then incident on the transmission diffraction grating, thereby further improving the light transmission quality.

[0024] In a fourth possible implementation manner of the second aspect, the sub-beam deflected by the optical deflection component is combined by the transmission diffraction grating and then output through the optical output port, so that the wavelength selective switch has the functions of wavelength separation and combination.

[0025] In a third aspect, the present application also provides an optical communication device including the wavelength selective switch of the second aspect. The optical communication device can be an optical switching node, specifically a reconfigurable optical add-drop multiplexer (ROADM). The ROADM can include multiple wavelength selective switches, which can be implemented by the wavelength selective switch of the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the architecture of the all-optical network system provided for this application;

[0027] Figure 2a A side view of a wavelength selective switch provided in one embodiment of the present application;

[0028] Figure 2b for Figure 2a A top view of the wavelength selective switch is shown;

[0029] Figure 2c A schematic diagram of a possible structure of a wavelength selective switch provided in one embodiment of the present application;

[0030] Figure 3 A schematic diagram of the stacked structure of a transmission diffraction grating provided in the first embodiment of the present application;

[0031] Figure 4a The incident light beam is in TE polarization mode. Figure 3 -1 order diffraction efficiency gradient diagram of the transmission diffraction grating shown;

[0032] Figure 4b The incident light beam is in TM polarization mode. Figure 3 -1 order diffraction efficiency gradient diagram of the transmission diffraction grating shown;

[0033] Figure 5 for Figure 3 Schematic diagram of the relationship between the -1 order diffraction efficiency of the transmission diffraction grating and the incident wavelength;

[0034] Figure 6 for Figure 3 Schematic diagram of the relationship between the -1 order diffraction efficiency of the transmission diffraction grating and the incident angle;

[0035] Figure 7 for Figure 3 Schematic diagram of the relationship between the incident angle and polarization loss of the transmission diffraction grating shown;

[0036] Figure 8 A schematic diagram of the stacked structure of a transmission diffraction grating provided in the second embodiment of the present application;

[0037] Figure 9a The incident light beam is in TE polarization mode. Figure 3 -1 order diffraction efficiency gradient diagram of the transmission diffraction grating shown;

[0038] Figure 9b The incident light beam is in TM polarization mode. Figure 3 -1 order diffraction efficiency gradient diagram of the transmission diffraction grating shown;

[0039] Figure 10 The -1 order diffraction efficiency of the transmission type diffraction grating shown in FIG. 1 1 versus the incident wavelength is shown in FIG. 1 2; Figure 8

[0040] Figure 11 The -1 order diffraction efficiency of the transmission type diffraction grating shown in FIG. 1 1 versus the incident angle is shown in FIG. 1 3; Figure 8

[0041] Figure 12 The incident angle versus the polarization loss of the transmission type diffraction grating shown in FIG. 1 1 is shown in FIG. 1 6; Figure 8

[0042] Figure 13 The stack structure of the transmission type diffraction grating provided by the third embodiment of the present application is shown in FIG. 1 9;

[0043] Figure 14a The -1 order diffraction efficiency gradient of the transmission type diffraction grating shown in FIG. 1 1 when a TE polarization mode incident light beam is incident is shown in FIG. 20; Figure 13

[0044] The -1 order diffraction efficiency gradient of the transmission type diffraction grating shown in FIG. 1 1 when a TM polarization mode incident light beam is incident is shown in FIG. 21 ; Figure 14b Figure 13 The -1 order diffraction efficiency of the transmission type diffraction grating shown in FIG. 1 1 versus the incident wavelength is shown in FIG. 22;

[0045] Figure 15 Figure 13 The -1 order diffraction efficiency of the transmission type diffraction grating shown in FIG. 1 1 versus the incident angle is shown in FIG. 23;

[0046] Figure 16 The incident angle versus the polarization loss of the transmission type diffraction grating shown in FIG. 1 1 is shown in FIG. 26. Figure 13

[0047] Figure 17 The incident angle versus the polarization loss of the transmission type diffraction grating shown in FIG. 1 1 is shown in FIG. 26. Figure 13 DETAILED DESCRIPTION

[0048] Please refer to Figure 1 ​​​​​​​The application provides an all optical network system (AON) 200, which comprises a backbone network ring 201 and an access network ring 203. The backbone network ring 201 comprises a plurality of optical switching nodes 2011 (also referred to as all optical nodes). The all optical network system 200 is a system based on dense wavelength division multiplexing technology, and the optical switching node 2011 adopts a reconfigurable optical add-drop multiplexer (ROADM) which can dynamically configure add or drop wavelengths as required, so as to realize flexible scheduling of services. The access network ring 203 comprises a plurality of user terminals 2031. The access network ring 203 transmits optical signals to the backbone network ring 201 and the backbone network ring 201 transmits optical signals to the access network ring 203 through the optical switching node 2011. The access network ring 203 refers to all devices between the backbone network ring 201 and the user terminal 2031. The user terminal 2031 can be a device providing voice and / or data connection to a user, can be connected to a computing device such as a laptop computer or a desktop computer, or can be a stand-alone device such as a personal digital assistant (PDA), without limitation.

[0049] The optical switching node 2011 adopting the ROADM comprises a plurality of wavelength selective switches 50. The wavelength selective switch 50 can realize arbitrary cross interconnection of optical signals between the access network ring 203 and the backbone network ring 201, and can dynamically adjust add or drop wavelengths of each optical switching node 2011 in the all optical network 200, so as to realize wavelength resource allocation between the optical switching nodes 2011 in the all optical network 200. It can be understood that the optical switching node 2011 adopting the ROADM can also comprise other modules, such as a fiber amplifier, a multicast switch (MCS), an arrayed waveguide grating, etc., without limitation.

[0050] More specifically, please refer to Figure 2a , Figure 2a A side view of the wavelength selective switch provided in an embodiment of the application is shown in FIG. 1. The wavelength selective switch 50 comprises an optical input port 51, an optical output port 52, a transmission type diffraction grating 53, an optical deflection component 54, a first lens group 55, a second lens group 56, a polarization optical component 57, and a beam shaping component 58.

[0051] The optical input port 51 is used to input an incident light beam, which includes optical signals of two or more wavelengths. The transmission-type diffraction grating 53 is used to spatially separate the incident light beam to form two or more sub-beams of different wavelengths. In other words, the transmission-type diffraction grating 53 is used to spatially separate the incident light beam with two or more wavelength optical signals into two or more single-wavelength optical signals (single-wavelength sub-beams) with a single wavelength. The optical deflection component 54 is used to selectively deflect a plurality of sub-beams of different wavelengths and switch them to the corresponding optical output port 52 for output. The first lens group 55, the second lens group 56 and the beam shaping component 58 are used to shape the light beam propagating in the wavelength selection switch 50 to improve the transmission quality of the optical signal in the wavelength selection switch 50. It can be understood that Figure 2a Only one optical input port 51 and one optical output port 52 are schematically shown in the figure. The number of optical input ports 51 may be two or more, and the number of optical output ports 52 may be two or more.

[0052] The incident beam (such as Figure 2a The incident light beam L) shown in FIG is incident on the polarization optical component 57 after passing through the light input port 51 and the first lens group 55 . After the polarization state of the incident light beam is adjusted by the polarization optical component 57 , it is incident on the transmission diffraction grating 53 .

[0053] Please refer to Figure 2b , Figure 2b for Figure 2a The incident light beam is spatially separated into sub-beams of different wavelengths (such as Figure 2b The M1...M N ), that is, wavelength division (also known as beam splitting). Sub-beams of different wavelengths pass through the second lens group 56 and the beam shaping component 58 before entering the light deflection component 54. The light deflection component 54 selectively deflects the sub-beams of different wavelengths, for example by changing the reflection or refraction angles of the sub-beams, thereby changing the propagation directions of the sub-beams.

[0054] Please refer again Figure 2a After being deflected by the optical deflection component 54, the sub-beams of different wavelengths pass through the beam shaping component 58 and the second lens group 56 and are incident on the transmission diffraction grating 53. The transmission diffraction grating 53 combines the sub-beams of different wavelengths (also called beam combining). The combined beam passes through the polarization optical component 57 and the first lens group 55 and is then transmitted to the optical output port 52 for output. Since the sub-beams of different wavelengths obtained after being deflected by the transmission diffraction grating 53 are deflected by the optical deflection component 54, then combined by the transmission diffraction grating 53 and output to the optical output port 52, the wavelength selective switch 50 has the functions of both deflection and combining.

[0055] The optical input port 51 and the optical output port 53 can each include a fiber port 513 and a collimating mirror 515. The fiber port 513 is used to externally connect an optical fiber. The collimating mirror 515 is located between the fiber port 513 and the first lens group 55. Each collimating mirror 515 corresponds to the fiber port 513 one-to-one and is used to perform collimating processing to improve the quality and efficiency of optical transmission in the wavelength selective switch 50.

[0056] It can be understood that the optical input port 51 and the optical output port 52 can omit the collimating mirror 515; the optical input port 51 and the optical output port 52 can be arranged in the same component, and the wavelength selective switch 50 folds the optical path by adding optical devices such as a curved reflecting component and the like, thereby reducing the occupied space of the wavelength selective switch 50.

[0057] It can be understood that the wavelength selective switch 50 can only have the function of wavelength division, and in another embodiment, referring to Figure 2c , the wavelength selective switch 50 includes an optical input port 51, an optical output port 52, a transmission type diffraction grating 53, and an optical deflection component 54. The optical input port 51 is used to input an incident light beam, and the incident light beam includes optical signals of two or more wavelengths. The transmission type diffraction grating 53 is used to spatially separate the incident light beam to form a plurality of sub-beams of different wavelengths, in other words, the transmission type diffraction grating 53 is used to spatially separate the incident light beam with a plurality of wavelength signals into a plurality of single-wavelength signals (single-wavelength sub-beams) with a single wavelength. For example, the incident light beam (L as shown in Figure 2c ) is input through the optical input port 51, the incident light beam has a plurality of wavelength signals of two or more wavelengths, and after passing through the transmission type diffraction grating 53, a plurality of spatially separated sub-beams of different wavelengths (M1, M N-1 , M N ) can be formed; after the modulation of the optical deflection component 54, for example, by changing the reflection angle or the refraction angle of each of the sub-beams (each single-wavelength signal included in the plurality of single-wavelength signals), the transmission direction of the sub-beams is changed, so that each sub-beam can be output to the desired optical output port 52.

[0058] It can be understood that the wavelength selective switch 50 can only have the function of wavelength combination, and in still another embodiment, the number of incident light beams is two or more, and the wavelengths of the two or more incident light beams can be the same or different. The incident light beams are incident through the optical input port 51, combined through the transmission type diffraction grating 53, and deflected by the optical deflection component 54 to the optical output port 52. It can be seen that the wavelength selective switch 50 can have the functions of wavelength division and / or wavelength combination according to the design of the internal optical path.

[0059] In order to improve the quality of the optical signals transmitted in the wavelength selective switch 50, some optical devices can be added as needed and arranged flexibly in the wavelength selective switch 50. The wavelength selective switch 50 can further include other optical devices, such as a lens group, a prism, an optical crystal, a polarization optical element, and the like. In an embodiment, the wavelength selective switch 50 further includes a prism and / or a mirror, so that the light beam can pass through the transmission type diffraction grating 53 multiple times, thereby improving the light splitting capability of the wavelength selective switch 50 and facilitating the miniaturization of the wavelength selective switch 50. The prism can be glued to the transmission type diffraction grating 53 to form an integral whole, so as to reduce the number of adjustment elements, compensate for the nonlinear dispersion effect, and the like.

[0060] The surface of the diffraction grating is usually provided with a plurality of grooves to form a plurality of grating ridges on the diffraction grating. The plurality of grooves are periodic structures of a sub-wavelength order. The optical signals of an incident light beam incident to the diffraction grating are subjected to diffraction by each groove of the diffraction grating and interference between the grooves, so that the amplitude and / or the phase of the optical signals are subjected to periodic spatial modulation, thereby splitting or combining the incident light beam.

[0061] When the optical signals are incident to the diffraction grating, different diffraction orders can be generated, satisfying the following grating equation, as shown in equation (1):

[0062] mλ = d(sinα ± sinθ) equation (1)

[0063] wherein m is a diffraction order, m is an integer, and the value of m is 0, ±1, ±2, …; λ is the wavelength of the incident optical signals; d is a grating constant (also referred to as a grating period), that is, the distance between two grooves of the diffraction grating, and the grating constant is also the reciprocal of the grating line density. The grating line density refers to the number of grooves per millimeter of the diffraction grating, and the unit is line / mm.

[0064] α is an incident angle, and θ is a diffraction angle. When the incident angle is the same as the diffraction angle, the diffraction grating has the highest efficiency, and the angle is referred to as a Littrow angle. The Littrow angle can be obtained by equation (2):

[0065] θ L = α = θ = arcsin(mλ / 2d) equation (2)

[0066] The grating equation, that is, equation (1), is differentiated to obtain a grating dispersion equation, as shown in equation (3). The dispersion capability of the diffraction grating is the capability of separating the exit angles of optical signals of different colors. As can be seen from equation (3), the dispersion capability of the diffraction grating is proportional to the grating line density and inversely proportional to the grating constant:

[0067] dθ / dλ = m / dcosθ equation (3)

[0068] The polarization state of the optical signal of an incident beam is typically described in terms of two orthogonal components. For one component, the electric field vector is perpendicular to the plane of incidence; this component is often denoted as "TE" polarization, and can alternatively be referred to as "S polarization." For the other component, the magnetic field vector is perpendicular to the plane of incidence; this component is often denoted as "TM" polarization, and can alternatively be referred to as "P polarization." In general, the polarization state can be fully characterized by the ratio of the TE to TM polarization components, as well as the phase between the TE and TM polarization components.

[0069] On the one hand, the polarization state of the optical signal is unpredictable when it is coupled into the wavelength selective switch from the optical fiber, and the diffraction grating tends to be very sensitive to the polarization state of the incident optical signal, so that the polarization loss (the difference of the optical insertion loss for different polarization states) of the wavelength selective switch will be very large. Thus, the diffraction grating in the wavelength selective switch is usually selected to be polarization independent. However, the polarization independent grating usually has a very deep grating structure, so that the fabrication of the diffraction grating is difficult. Because the polarization independent grating has a very deep grating structure, the thickness of the diffraction grating is large, so that it is not conducive to the miniaturization of the diffraction grating and the wavelength selective switch.

[0070] On the other hand, the all-optical network system using dense wavelength division multiplexing technology also requires the diffraction grating of the wavelength selective switch to have high diffraction efficiency (usually more than 90%) in the optical fiber communication band (such as the optical communication C band [1524, 1575] nm) to reduce the optical loss in the wavelength selective switch. The diffraction efficiency and polarization response characteristics of the diffraction grating are often sensitive to the incident angle, and the diffraction efficiency of the diffraction grating will decrease when the incident angle deviates from the Littrow angle. The optical signals with different wavelengths in the incident beam have different incident angles in the diffraction grating.

[0071] Based on this, the application provides a transmission type diffraction grating applied to the C wave band [1524, 1575] nm of optical communication, which has high diffraction efficiency and polarization independence in a wide range of incident angles near the Littrow angle. The transmission type diffraction grating comprises a transparent substrate and a grating structure arranged in layers, and the grating structure comprises at least two grating layers. The bottommost grating layer of the grating structure on the transparent substrate is made of a high refractive index material, each grating layer except the bottommost grating layer is made of a high refractive index material or a low refractive index material, the materials of the two adjacent grating layers are different, the refractive index range of the low refractive index material is [1.30, 1.80], the refractive index range of the high refractive index material is [1.90, 2.50], the thickness range of the grating layer made of the low refractive index material is [100, 400] nm, the total thickness range of the grating structure is [1000, 1450] nm, the duty cycle range of the grating structure is [50%, 60%], and the line density of the grating structure is not less than 965 lines / mm. The duty cycle refers to the ratio of the width of the grating ridge to the grating period in one grating period.

[0072] The materials of the two adjacent grating layers are different, including two cases: the materials of the two adjacent grating layers are different but the refractive index characteristics are the same; and the materials of the two adjacent grating layers are different and the refractive index characteristics are different. In other words, the refractive index between adjacent grating layers is different.

[0073] Since the total thickness range of the grating structure is [1000, 1450] nm, the thickness range of the grating layer made of the low refractive index material is [100, 400] nm, the bottommost grating layer of the grating structure near the transparent substrate is made of the high refractive index material, and for the incident light beams of TE and TM polarization modes in the C wave band [1524, 1575] nm of optical communication, the incident angle is in the range of [θ-5.5°, θ+5.5°], wherein θ is the Littrow angle, the lowest diffraction efficiency of the transmission type diffraction grating-1 is greater than 92%, and the highest polarization loss is less than 0.2 dB, that is, the diffraction efficiency of the incident light signal of TE and TM polarization modes is less than 0.2 dB. In addition, compared with the total thickness of the conventional grating structure which is greater than 1700 nm, the total thickness range of the grating structure of the application is [1000, 1450] nm, which effectively reduces the thickness and groove depth of the transmission type diffraction grating, and is beneficial to the miniaturization of the transmission type diffraction grating and the reduction of the manufacturing difficulty of the transmission type diffraction grating.

[0074] It can be understood that each grating layer can be decomposed into a plurality of equivalent refractive index sub-grating layers, for example, a plurality of sub-grating layers composed of medium materials with similar refractive indices.

[0075] It can be understood that the at least two grating layers include two or more grating layers, for example, four, five, six or more grating layers.

[0076] The transmission diffraction grating is further described below in detail.

[0077] Referring to Figure 3 The transmission diffraction grating 53 provided by the first embodiment of the present application includes a transparent substrate 531 and a grating structure 535 arranged in a stack. The grating structure 535 includes a first grating layer 5351 and a second grating layer 5353, and the second grating layer 5353 is sandwiched between the first grating layer 5351 and the transparent substrate 531. The transparent substrate 531 is made of fused quartz. The first grating layer 5351 is made of a low refractive index material, and the second grating layer 5353 is made of a high refractive index material. The refractive index of the low refractive index material ranges from 1.30 to 1.80, and the refractive index of the high refractive index material ranges from 1.90 to 2.50. The high refractive index material includes one of TiO2, Nb2O5 and Ta2O5, and the low refractive index material includes one of SiO2, Al2O3, CaF2, LiF, MgF2, NaF, SrF2 and cryolite. The duty cycle of the grating structure 53 ranges from 50% to 60%, and the line density of the grating structure 53 is not less than 965 lines / mm, so as to improve the dispersion capability of the transmission diffraction grating 53.

[0078] The thickness of the first grating layer 5351 ranges from 240 nm to 330 nm, and the thickness of the second grating layer 5353 ranges from 1000 nm to 1120 nm. Compared with the grating structure of a conventional transmission diffraction grating (usually with a thickness ranging from 1700 nm to 2000 nm), the grating structure 535 provided by the first embodiment has a smaller thickness, thereby reducing the etching difficulty of the grating structure 53. Since each grating layer has a tolerance, the cumulative tolerance is larger when the number of grating layers is larger. Compared with a conventional grating structure with more than three layers, the grating structure 535 provided by the first embodiment of the present application has only two layers, thereby reducing the cumulative tolerance between the layers and improving the diffraction efficiency and light transmission quality of the transmission diffraction grating 53.

[0079] The grating structure 535 is provided with a plurality of grooves 501 penetrating the first grating layer 5351 and the second grating layer 5353, so as to form a plurality of grating ridges 503 on the grating structure 535. The grooves 501 are used for diffracting the incident light beam L. Figure 3 As shown in FIG. 2, the width of the grating ridge 503 is w, the grating constant is d, and the ratio of w to d is the duty cycle of the transmission diffraction grating 53.

[0080] As shown in FIG. 2, the width of the grating ridge 503 is w, the grating constant is d, and the ratio of w to d is the duty cycle of the transmission diffraction grating 53. Figure 3As shown, the incident light beam L is incident from the first grating layer 5351, passes through the second grating layer 5353, and then exits from the transparent substrate 531. The incident light beam L includes two or more wavelength light signals. The incident light beam L is separated into sub-beams of different wavelengths by the transmission diffraction grating 53 ( Figure 3 It is understood that the incident light beam L may also be incident from the side of the transparent substrate 531 , pass through the second grating layer 5353 , and then be emitted from the first grating layer 5351 .

[0081] Using a rigorous coupled wave theory model, we can obtain a typical gradient diagram of the diffraction efficiency of the TE and TM polarized incident light signals versus the thickness of the two-layer grating structure (including the first and second grating layers). Figure 4a and Figure 4b , it can be found that when the thickness of the first grating layer 5351 is [240, 330] nm and the thickness of the second grating layer 5353 is [1000, 1120] nm, the diffraction efficiency of the incident light beams in both TE and TM polarization modes can reach greater than 96% at the same time, and the difference in the diffraction efficiency of the incident light beams in the two polarization modes is small.

[0082] See also Figure 5 In the optical communication C-band [1524, 1575] nm, for TE and TM polarization mode incident light beams with an incident angle of 48°, the -1st order diffraction efficiency of the transmission type diffraction grating 35 exceeds 96%.

[0083] See also Figure 6 For incident light beams with an incident angle within the range of [θ-5.5°, θ+5.5°], the -1 order diffraction efficiency of the transmission type diffraction grating 53 exceeds 96%, where θ is the Littrow angle. Figures 5-6 The solid line in FIG. 5 is the diffraction efficiency of the transmission type diffraction grating 53 when the TE polarized incident light beam is incident, and the dotted line is the diffraction efficiency of the transmission type diffraction grating 53 when the TM polarized incident light beam is incident.

[0084] See also Figure 7 For an incident light beam with an incident angle within the range of [θ-5.5°, θ+5.5°], the polarization dependent loss (PDL) of the transmission diffraction grating 53 is less than 0.1 dB.

[0085] See also Figure 8The transmissive diffraction grating 53 provided in the second embodiment of the present application includes a transparent substrate 531 and a grating structure 535, which are stacked. The grating structure 535 includes a first grating layer 5351, a second grating layer 5353, and a third grating layer 5355, which are arranged in sequence. The third grating layer 5355 is sandwiched between the transparent substrate 531 and the second grating layer 5353. The third grating layer 5355 is made of a high-refractive-index material, the second grating layer 5353 is made of a low-refractive-index material, and the first grating layer 5351 is made of a high-refractive-index material. The refractive index of the low-refractive-index material is in the range of [1.30, 1.80], and the refractive index of the high-refractive-index material is in the range of [1.90, 2.50]. The high-refractive-index material includes one of TiO2, Nb2O5, Ta2O5, and Si3N4; the low-refractive-index material includes one of SiO2, Al2O3, CaF2, LiF, MgF2, NaF, SrF2, and cryolite. The duty cycle of the grating structure 53 ranges from 50% to 60%, and the line density of the grating structure 533 is no less than 965 lines / mm. The grating structure 535 consists of only three grating layers, reducing the cumulative tolerance between the stacked layers of the grating structure 535 and further improving the diffraction efficiency of the transmission-type diffraction grating 535.

[0086] The thickness of the first grating layer 5351 is in the range of [20, 45] nm, the thickness of the second grating layer 5353 is in the range of [150, 230] nm, and the thickness of the third grating layer 5355 is in the range of [1030, 1090] nm. Compared to the grating structure of a conventional transmission-type diffraction grating (typically having a thickness greater than 1700 nm), the grating structure 535 provided in the second embodiment is thinner, reducing the difficulty of etching the grating structure 53. Furthermore, the grating structure 535 consists of only three layers, reducing the tolerance between the stacked layers and facilitating improved diffraction efficiency and light transmission quality of the transmission-type diffraction grating 53.

[0087] The grating structure 535 is provided with grooves 501 penetrating the first grating layer 5351, the second grating layer 5353 and the third grating layer 5355, for diffracting the incident light beam L. Figure 8 As shown, the incident light beam L is incident from the first grating layer 5351, passes through the second grating layer 5353 and the third grating layer 5355, and then exits from the transparent substrate 531. The incident light beam L includes two or more wavelength light signals. The incident light beam L is separated into sub-beams of different wavelengths by the transmission diffraction grating 53 ( Figure 8 It is understood that the incident light beam L may also be incident from the side of the transparent substrate 531 , pass through the third grating layer 5355 and the second grating layer 5353 , and then be emitted from the first grating layer 5351 .

[0088] The diffraction efficiency of TE and TM polarized incident light beams and the typical gradient of the thickness of the three-layer grating structure (including the first grating layer 5351, the second grating layer 5353, and the third grating layer 5355) are obtained by using the rigorous coupled wave theory model optimization. Please refer to Figure 9a With Figure 9b When the thickness of the first grating layer 5351 is 30 nm, the thickness of the second grating layer 5353 is [150, 230] nm, and the thickness of the third grating layer 5355 is [1030, 1090] nm, the diffraction efficiency of TE and TM polarized incident light beams can simultaneously reach more than 96%, and the diffraction efficiency difference of the two polarizations is small.

[0089] Please refer to Figure 10 In the range of the optical communication C waveband [1524, 1575] nm, for TE and TM polarized incident light beams with an incident angle of 48°, the -1 order diffraction efficiency of the transmission diffraction grating 35 is more than 96%.

[0090] Please refer to Figure 11 For incident light beams with an incident angle in the range of [θ-5.5°, θ+5.5°], where θ is the Littrow angle, the -1 order diffraction efficiency of the transmission diffraction grating 35 is more than 96%. It should be noted that, Figures 10-11 The solid line in FIG. 5 is the diffraction efficiency of the transmission diffraction grating 53 when TE polarized incident light is incident, and the dashed line is the diffraction efficiency of the transmission diffraction grating 53 when TN polarized light is incident.

[0091] Please refer to Figure 12 For incident light beams with an incident angle in the range of [θ-5.5°, θ+5.5°], the polarization dependent loss (PDL) of the transmission diffraction grating 53 is less than 0.1 dB.

[0092] In some embodiments, if the refractive index of the high refractive index material is between [1.90, 2.00], and the number of grating layers of the grating structure 535 is greater than or equal to 3, the type of high refractive index material is more than one.

[0093] It can be understood that the grating structure can include more than three grating layers, provided that the following conditions are met: the materials of the two adjacent grating layers are different; at least one of the at least two grating layers is made of a high refractive index material; at least one of the at least two grating layers is made of a low refractive index material; the refractive index of the low refractive index material ranges from 1.30 to 1.80; the refractive index of the high refractive index material ranges from 1.90 to 2.50; the thickness of the grating layer made of the low refractive index material ranges from 100 nm to 400 nm; the grating layer closest to the bottom layer of the transparent substrate is made of the high refractive index material; the total thickness of the grating structure ranges from 1000 nm to 1450 nm; the duty cycle of the grating structure ranges from 50% to 60%; and the line density of the grating structure is not less than 965 lines / mm. The following is a simple example. The grating structure includes a first grating layer, a second grating layer, a third grating layer, and a fourth grating layer arranged in sequence. The fourth grating layer is between the third grating layer and the transparent substrate, and is made of a high refractive index material. The materials of the first grating layer and the second grating layer are different, the materials of the second grating layer and the third grating layer are different, and the materials of the third grating layer and the fourth grating layer are different. One of the first grating layer, the second grating layer, and the third grating layer is made of a low refractive index material. For example, the fourth grating layer is made of a high refractive index material (such as Si3N4), the third grating layer is made of a low refractive index material Al2O3, the second grating layer is made of a high refractive index material (such as TiO2), and the first grating layer is made of a low refractive index material (such as MgF2). The thickness of the third grating layer and the first grating layer ranges from 100 nm to 400 nm. The total thickness of the grating structure ranges from 1000 nm to 1450 nm. The duty cycle of the grating structure ranges from 50% to 60%. The line density of the grating structure is not less than 965 lines / mm.

[0094] The transmission diffraction grating provided by the third embodiment of the present application has substantially the same structure as the transmission diffraction grating provided by the second embodiment, except that Figure 13The second grating layer 5353 is made of a high-refractive-index material. That is, both the second grating layer 5353 and the third grating layer 5355 are made of a high-refractive-index material. The first grating layer 5351 is made of a low-refractive-index material, and the refractive index of the second grating layer 5353 is different from that of the third grating layer 5351. The refractive index of the low-refractive-index material ranges from [1.30 to 1.80], and the refractive index of the high-refractive-index material ranges from [1.90 to 2.50]. The high-refractive-index material includes one of TiO2, Nb2O5, Ta2O5, and Si3N4, while the low-refractive-index material includes one of SiO2, Al2O3, CaF2, LiF, MgF2, NaF, SrF2, and cryolite. The duty cycle of the grating structure 53 ranges from [50% to 60%], and the line density of the grating structure 53 is no less than 965 lines / mm.

[0095] The thickness range of the first grating layer 5351 is [200, 350] nm, the thickness range of the second grating layer 5353 is [420, 620] nm, and the thickness range of the third grating layer 5355 is [485, 720] nm.

[0096] Two adjacent grating layers are made of different materials, which includes two situations. The first situation is that the two adjacent grating layers are made of different materials but have the same refractive index characteristics. For example, the second grating layer 5353 and the third grating layer 5355 are both made of high-refractive-index materials. The second grating layer 5353 and the third grating layer 5355 are made of different materials, but have the same refractive index characteristics. The second situation is that the two adjacent grating layers are made of different materials and have different refractive index characteristics. For example, the second grating layer 5353 and the third grating layer 5355 are both made of high-refractive-index materials. The second grating layer 5353 and the third grating layer 5355 are made of different materials, and have different refractive index characteristics.

[0097] The grating structure 53 is provided with grooves 501 penetrating the first grating layer 5351, the second grating layer 5353 and the third grating layer 5355, for diffracting the incident light beam L. Figure 13 As shown, the incident light beam L is incident from the first grating layer 5351, passes through the second grating layer 5353 and the third grating layer 5355, and then exits from the transparent substrate 531. The incident light beam L includes two or more wavelength light signals. The incident light beam L is separated into sub-beams of different wavelengths by the transmission diffraction grating 53 ( Figure 13 It is only illustratively shown that a sub-beam is emitted. It can be understood that the incident light beam L can also be incident from the side of the transparent substrate 531, pass through the third grating layer 5355 and the second grating layer 5353, and then be emitted from the first grating layer 5351.

[0098] The rigorous coupled-wave theory model is used for optimization, and the typical gradient diagram of the diffraction efficiency of TE and TM polarized incident light beams and the thickness of the three-layer grating structure (including the first grating layer 5351, the second grating layer 5353, and the third grating layer 5355) is obtained, please refer to Figure 14a With Figure 14b When the thickness of the first grating layer 5351 is 300 nm, the thickness of the second grating layer 5353 is [420, 620] nm, and the thickness of the third grating layer 5355 is [485, 720] nm, the diffraction efficiency of TE and TM polarized incident light beams can reach more than 96% at the same time, and the diffraction efficiency difference of the two polarizations is small.

[0099] Please refer to Figure 15 In the range of the optical communication C waveband [1524, 1575] nm, for TE and TM polarized incident light beams with an incident angle of 48°, the -1 order diffraction efficiency of the transmission diffraction grating 35 is more than 96%.

[0100] Please refer to Figure 16 For incident light beams with an incident angle in the range of [θ-5.5°, θ+5.5°], where θ is the Littrow angle, the -1 order diffraction efficiency of the transmission diffraction grating 35 is more than 96%. It should be noted that, Figures 15-16 The solid line in FIG. 5 is the diffraction efficiency of the transmission diffraction grating 53 when TE polarized incident light is incident, and the dashed line is the diffraction efficiency of the transmission diffraction grating 53 when TN polarized light is incident.

[0101] Please refer to Figure 17 For incident light beams with an incident angle in the range of [θ-5.5°, θ+5.5°], the polarization dependent loss (PDL) of the transmission diffraction grating 53 is less than 0.1 dB.

[0102] The above merely illustrates the specific embodiments of the present application, but the protection scope of the present application is not limited to this. Any person skilled in the art can easily think of changes or replacements within the technical range disclosed in the present application, which should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A polarization independent transmissive diffraction grating, characterized in that, The grating structure includes a transparent substrate and a grating structure arranged in a stack, the grating structure includes at least two grating layers, wherein the bottommost grating layer of the grating structure on the transparent substrate is made of a high refractive index material, each grating layer except the bottommost grating layer is made of a high refractive index material or a low refractive index material, the materials of two adjacent grating layers are different, the refractive index of the low refractive index material ranges from 1.30 to 1.80, the refractive index of the high refractive index material ranges from 1.90 to 2.50, the thickness of the grating layer made of the low refractive index material ranges from 100 to 400 nm, the total thickness of the grating structure ranges from 1000 to 1450 nm, the duty cycle of the grating structure ranges from 50% to 60%, and the line density of the grating structure is not less than 965 lines / mm, wherein the -1 order diffraction efficiency of the transmission diffraction grating is more than 92% and the polarization loss of the transmission diffraction grating is less than 0.2 dB for an incident light beam with an incident angle in the range of Littrow angle ± 5.5°, and the incident light beam is a C-band [1524, 1575] nm light beam for TE and TM polarization modes.

2. The transmissive diffraction grating according to claim 1, wherein The grating structure includes a first grating layer and a second grating layer, and the second grating layer is arranged between the first grating layer and the transparent substrate, the thickness of the first grating layer ranges from 240 to 330 nm, and the thickness of the second grating layer ranges from 1000 to 1120 nm.

3. The transmissive diffraction grating according to any one of claims 1 to 2, wherein The high refractive index material includes one of TiO2, Nb2O5, and Ta2O5, and the low refractive index material includes one of SiO2, Al2O3, CaF2, LiF, MgF2, NaF, SrF2, and cryolite.

4. The transmissive diffraction grating of claim 1, wherein, The grating structure includes a first grating layer, a second grating layer, and a third grating layer arranged in sequence, and the third grating layer is arranged between the transparent substrate and the second grating layer, the second grating layer is made of the low refractive index material, and the first grating layer is made of the high refractive index material.

5. The transmissive diffraction grating according to claim 4, wherein The thickness of the first grating layer ranges from 20 to 45 nm, the thickness of the second grating layer ranges from 150 to 230 nm, and the thickness of the third grating layer ranges from 1030 to 1090 nm.

6. The transmissive diffraction grating of claim 1, wherein, The grating structure includes a first grating layer, a second grating layer, and a third grating layer arranged in sequence, and the third grating layer is arranged between the transparent substrate and the second grating layer, the second grating layer is made of the high refractive index material, and the first grating layer is made of the low refractive index material, and the third grating layer and the second grating layer are made of different materials.

7. The transmissive diffraction grating according to claim 6, wherein The thickness of the first grating layer ranges from 200 to 350 nm, the thickness of the second grating layer ranges from 420 to 620 nm, and the thickness of the third grating layer ranges from 485 to 720 nm.

8. The transmissive diffraction grating according to any one of claims 4 to 7, wherein The high-refractive-index material comprises one of TiO2, Nb2O5, Ta2O5, Si3N4, and the low-refractive-index material comprises one of SiO2, Al2O3, CaF2, LiF, MgF2, NaF, SrF2, and cryolite.

9. The transmissive diffraction grating according to any one of claims 1 to 8, characterized in that, The transparent substrate comprises fused quartz.

10. A wavelength selective switch, characterized by, The wavelength selective switch comprises the light input port, the light output port, the light deflection component, and the transmissive diffraction grating according to any one of claims 1-9, the light input port is used for inputting an incident light beam, the transmissive diffraction grating is used for splitting or combining the incident light beam, and the light deflection component is used for selectively deflecting the light beam obtained after the splitting or combining by the transmissive diffraction grating to switch to a corresponding light output port for output.

11. An optical communication device, comprising: The wavelength selective switch comprises the wavelength selective switch according to claim 10.

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