CN101738663A - Wavelength-Independent Fused Silica Transmission Polarization Beamsplitter Gratings - Google Patents

Abstract

A wavelength-independent fused silica transmission polarization beam splitter grating for use in the ultraviolet to near-infrared band with a wavelength range from 300 nanometers to 1800 nanometers, characterized in that the incident conditions and normalized structural parameters do not change with wavelength. The normalized structural parameters are: the duty cycle of the grating is 0.5, the ratio of the incident wavelength to the grating period is 1.74-1.75, and the ratio of the etching depth to the grating period is 2.21-2.22. In the above-mentioned band, the grating has extremely high transmission efficiency and polarization extinction ratio. The actual production of the wavelength-independent fused silica transmission polarization beam splitter grating of the present invention can be processed by optical holographic recording technology or electron beam direct writing device combined with microelectronic deep etching technology. The process is mature, the cost is low, and it can be mass-produced, which has important practical prospects.

Description

Wavelength-Independent Fused Silica Transmission Polarization Beamsplitter Gratings

technical field

The invention relates to a polarization beam-splitting grating, in particular to a wavelength-independent fused silica transmission polarization beam-splitting grating in the ultraviolet to near-infrared band.

Background technique

Gratings are widely used in various optical systems, and one of the important uses is as a beam splitting device in holographic systems, optical information processing systems and measurement systems. Traditional beam splitters based on multi-layer dielectric films have large energy losses, complex manufacturing processes, and high costs. Photonic crystals, which have emerged in recent years as beam splitters, also have disadvantages such as high cost and difficult manufacture. Fused silica is a very good optical material. It has a wide transmission spectrum from deep ultraviolet to far infrared, high optical quality, good temperature stability, high laser damage threshold, and small dispersion coefficient. Using fused silica as material, low-loss polarization-dependent, high-diffraction-efficiency gratings and polarization beam-splitting gratings have been designed and fabricated. Some literatures have reported high-density phase gratings as polarization beam splitters, but designers usually design for a single wavelength. When the incident wavelength is changed, the designed grating often does not meet the requirements and needs to be redesigned. Therefore, if the physical characteristics of fused silica can be used to design a polarization beam-splitting grating with a wavelength-independent structure, the designer does not need to repeat the design process when the wavelength changes, which will be of great practical application value.

The high-density rectangular deep-etched grating is a grating with a deep groove shape processed on the substrate by using the microelectronic deep-etch process. Due to the deep etching depth of the surface-etched grating, the diffraction performance is similar to that of a volume grating, which has the Bragg diffraction effect of a volume grating, which is completely different from the ordinary shallow-etched planar grating. The diffraction theory of high-density rectangular deep-etched gratings cannot be explained by simple scalar grating diffraction equations, but must use Maxwell's equations in vector form combined with boundary conditions to accurately calculate the results through coded computer programs. Moharam et al. have given the algorithm of strict coupled wave theory [Prior Art 1: M.G.Moharam et al., J.Opt.Soc.Am.A.12, 1077(1995)], which can solve this kind of high-density grating Diffraction problem, but this method can only calculate the corresponding diffraction efficiency for a single wavelength. As far as we know, so far, no one has given the design parameters of wavelength-independent deep-etched fused silica transmission polarization beam-splitting gratings in the ultraviolet to near-infrared band from 300 nm to 1800 nm.

Contents of the invention

The technical problem to be solved by the present invention is to provide a wavelength-independent fused silica transmission polarization beam-splitting grating in the ultraviolet to near-infrared band from 300 nanometers to 1800 nanometers, and the grating is required to have high transmittance and High polarization extinction ratio.

Technical solution of the present invention is as follows:

A wavelength-independent fused silica transmission polarization beam-splitting grating used in the ultraviolet to near-infrared band of 300 nm to 1800 nm is characterized in that: the normalized structural parameters of the grating and the incident conditions of the grating do not change due to the wavelength. The normalized structural parameters are: the duty ratio of the grating is 0.5, the ratio of the incident wavelength to the grating period is 1.74-1.75, and the ratio of the etching depth of the grating groove to the grating period is 2.21-2.22.

The basis of the present invention is as follows:

Figure 1 shows the geometry of a high-density rectangular deep-etched quartz grating. Regions 1 and 2 are both homogeneous, being respectively air (refractive index n ₁ =1) and fused silica (refractive index n ₂ , which varies with the incident wavelength). The grating vector K lies in the plane of incidence. TE polarization corresponds to the vibration direction of the electric field vector perpendicular to the incident plane, and TM polarized light corresponds to the vibration direction of the magnetic field vector perpendicular to the incident plane. When the incident light contains both TE and TM polarizations in a Littrow configuration (which can be expressed as ), the incident angle depends on the normalized parameter (λ/Λ)) when it is incident on the grating surface, the grating can mainly diffract the TE polarized light to the first-order transmission direction, and mainly diffract the TM polarized light to the zero-order transmission direction .

Under the grating structure shown in Figure 1, the present invention utilizes the improved grating simplified mode theory [prior art 2: I.C.Botten et al., Opt.Acta, 28, 413-428 (1981)], and adopts normalization Optimized design parameters to guide the design. The specific parameters are: the grating duty ratio, the ratio of the incident wavelength to the grating period, and the ratio of the etching depth to the grating period. In the present invention, the strict coupled wave theory [Prior Technology 1] is used to calculate the diffraction efficiencies of high-density deep-etched fused silica gratings in the 0-order and -1-order transmission directions respectively, combined with the simulated annealing algorithm [Prior Technology 3: W.Goffe et al., J.Econom., 60, 65-99 (1994)] for optimal search. The final optimization results show that when the duty ratio of the grating is 0.5 and the incidence is configured by Littrow, if the normalized structural parameters satisfy the ratio of the incident wavelength to the grating period is 1.74-1.75, and the ratio of the etching depth to the grating period is 2.21 ~2.22, the grating can achieve excellent polarization beam splitting effect in the ultraviolet to near-infrared band from 300 nm to 1800 nm. Specifically, the TM polarized light in the 0th order transmission direction and the TE polarized light in the -1st order transmission direction have high diffraction efficiency, while the diffraction efficiency of the TE polarized light in the 0th order transmission direction and the TM polarized light in the -1st order transmission direction is close to zero. For example, a typical example in the range of parameters required by the present invention is shown in the wavelength range of 300 nm to 1800 nm, as shown in Figure 2: the diffraction efficiency of TE polarized light in the 0th order transmission direction is 0.06% to 0.86%, TM The diffraction efficiency of polarized light is 97.5%-98.97%; the diffraction efficiency of TE polarized light in the -1 order transmission direction is 84.96%-89.4%, and the diffraction efficiency of TM polarized light is 0.0004%-0.57%. The extinction ratios of the 0-order and -1-order transmission directions are respectively defined as:

The extinction ratios of the above examples in the same wavelength range are shown in Fig. 3 . It can be seen that in such a wide range of wavelengths, the extinction ratio of the 0th order and the -1st order transmission direction is always higher than 20db, and the extinction ratio of the -1st order transmission direction increases with the increase of the wavelength. Especially when the wavelength is 1064 nm, the extinction ratios of the 0th order and -1st order transmission directions are equal, both are 35.95db. Because the new normalized structural parameters are adopted in the present invention and no specific wavelength is involved, the problem that the polarization beam splitting grating needs to be redesigned in the past under the incident conditions of different wavelengths is solved. And it has such a high extinction ratio in such a wide spectral range, which makes the present invention a very ideal polarization beam splitting device. At the same time, due to its simple rectangular grating structure, it is very easy to manufacture, so that the grating has important practical significance.

Description of drawings

Fig. 1 is the geometric structure of the wavelength-independent fused silica transmission polarization beam-splitting grating of the present invention.

In the figure, 1 represents area 1 (refractive index n ₁ ), 2 represents area 2 (refractive index n ₂ ), 3 represents grating, 4 represents incident light, 5 represents 0-order diffracted light, and 6 represents -1-order diffracted light , the two polarization directions are TE and TM respectively, Λ represents the spatial period of the grating, h represents the groove depth of the grating, and b represents the width of the protrusion of the grating (duty ratio f=b/Λ).

Fig. 2 is the curve (incident with Littrow configuration) of TE and TM polarized light of an example in the transmission direction of 0th order and -1st order transmission direction as a function of wavelength within the scope of the requirements of the present invention

Figure 3 is the curve of the polarization extinction ratio versus wavelength in the 0th order and -1st order transmission directions of the example in Figure 2

Detailed ways

Using micro-optical technology to fabricate high-density deep-etched rectangular gratings, first deposit a layer of metal chromium film on a dry and clean fused silica substrate, and evenly coat a layer of positive photoresist on the chromium film (Shipley, S1818, USA) . Then the grating is recorded by holographic recording, developed, and then the photoresist pattern is transferred from the photoresist to the chrome film with a chrome removal solution, and the excess photoresist is removed by chemical reagents. Finally, put the sample into an inductively coupled plasma etching machine for a certain period of plasma etching, transfer the grating to the fused silica substrate, and then remove the remaining chromium film with a chromium-removing solution to obtain high-density deep etching Fused silica grating with surface relief structure.

In the process of making the grating, under different incident wave conditions, the corresponding fused silica transmission polarization beam-splitting grating can be made by selecting the appropriate grating period and etching depth according to the normalized design parameters given by the present invention.

The wavelength-independent fused silica transmission polarization beam splitting grating of the invention has the characteristics of high diffraction efficiency, high extinction ratio and high laser damage threshold, and is a very ideal polarization beam splitting device. Due to the adoption of normalized parameter design, it is not aimed at a single incident wavelength, and has universal applicability in the selected wavelength band of the present invention, and its structure is simple and easy to implement. Using holographic grating recording technology or electron beam direct writing device combined with microelectronic deep etching process, it can be produced in large quantities and at low cost. The etched grating has stable and reliable performance, and has important practical prospects.

Claims (1)

1. one kind is used for 300 nanometers arrive near-infrared band to the ultraviolet of 1800 nanometers Wavelength irrelevant fused quartz transmission polarization beam-splitting grating, it is characterized in that: the normalized structural parameters of this grating are that dutycycle is 0.5, and the incident wavelength and the ratio in grating cycle are 1.74～1.75, the etching depth and the ratio in grating cycle of grating groove are 2.21～2.22.

Images