Fabrication of an ultra-narrow band-pass filter with 60 pm bandwidth in green light band

WANG Kai-xuan，CHEN Gang，LIU Ding-quan ，MA Chong，ZHANG Qiu-yu,3

（1. Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China；2. School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031, China；3. University of Chinese Academy of Sciences, Beijing 100049, China）

Abstract: Owing to the strong penetrating ability in the atmosphere, 532 nm-wavelength green laser has wide applications including free-space optical communications and laser three-dimensional mapping. A spectral filter, with a half-power bandwidth of less than 100 pm, is an important optical element to suppress the interference of background light. Therefore, an ultra-narrow band-pass filter based on optical interference film is designed and fabricated in this paper. The high and low refractive index film are made of tantalum pentoxide(Ta2O5) and silicon dioxide (SiO2), respectively. The designed optical thin films are deposited on a fused quartz substrate by double-ion-beam sputtering deposition method. The transmission spectra of the filters are measured by a tunable laser and a power meter. The half-power bandwidths of the filters are (60±2) pm, and the transmittance reaches 62.6%.

Key words: optical thin film; thin film filter; picometer bandwidth; green light band; space laser mapping

1 Introduction

The green laser with 532 nm wavelength has strong penetrating ability in the atmosphere, and the corresponding light source and photoelectric receiver have stable performance. The laser with this wavelength has a good application prospect in laser lidar, free-space optical communications, space laser remote sensing, 3D mapping imaging and other fields[1-4]. In order to reduce the interference of background light, especially the strong influence of solar radiation, a spectral filter with passband width below 100 pm (i.e. 0.1 nm) is needed to suppress the background light[5]. For such a filter, the main filtering techniques that can be adopted include: acoustoptic modulation, atomic filtering, Fabry-Perot (FP) etalon, thin film interference filtering, etc.[6-8].

For spaceborne optical instruments (especially those facing a deep space flight), reliability, high optical efficiency and light weight are the key factors to be considered, so the F-P etalon and thin film interference are the main technical options. E.Troupaki[5]et al. constructed a spectral filter by using the F-P etalon technique, and achieved 30 pm bandwidth spectral filtering on American ICESat-2 satellite for the elevation mapping of snow and ice,clouds and land. The instrument uses 6 laser beams for mapping. ICESat-2 is the only altitude-mapping satellite currently in use abroad. This kind of spectral filter has high requirements for temperature control. Moreover, it is difficult to arrange many spectral filters when quite a lot of laser beams are needed. In order to develop the space elevation mapping with more laser beams, we designed and fabricated a bandpass filter with a target bandwidth of 60 pm based on precision optical thin film.

2 Design and fabrication of film system

Several related subnano-level spectral filtering techniques are listed in Table 1, each of which has its own characteristics. In this paper, the spectral filtering technique based on thin film interference is adopted.

Tab. 1 Comparison of main filtering techniques for sub-nanometer spectrum in visible light band表1 可見光波段的主要亞納米光譜濾波技術比較

2.1 Design of filtering film system

For the band-pass filter with very narrow halfpower bandwidth, the F-P structure with a single resonator is adopted to form a filter with all-medium film layer considering the actual deposition error of each layer of film. Its waveform is comparable to that of a filter based on F-P etalon, and its spectrum rectangularity can be improved by increasing the interference order of spacer layer. Ta2O5is selected as the high-index film layer and is applicable to ultra-narrow band-pass filters, such as the Dense Wavelength Division Multiplexing (DWDM)filter used in 4G and 5G optical communications,due to its excellent physical and chemical stability and transparency in near-ultraviolet, visible and short-wave infrared bands[9-11]. SiO2is selected as the low-index film layer, which has good transparency, high physical and chemical stability and good thermal matching with the substrate made of fused quartz due to the sharing of the same material.Fused quartz (JGS-1, 4 mm thick) is selected as the substrate of the ultra-narrow band-pass filter due to its good thermal stability[12].

The filtering film system is designed as Sub./(1H 1L)101H 2L 1H (1L 1H)101L / Air, where H and L respectively represent the Ta2O5and SiO2film layers with an optical thickness of 1/4 central wavelength, which is the laser wavelength of 532 nm. The low-index film layer 2L is designed as a spacer, since it has smaller linear expansion coefficient and refractive-index temperature coefficient,so has more stable filter spectrum compared with 2H spacer. Of course, this film system is also more sensitive to the incident angle. This is a Fabry-Perot structure with a single resonator, consisting entirely of dielectric films.

The passband width can be approximated by Equation (1)[13-14]:

wherenHandnLare respectively the refractive indices of the high and low index film layers,ngis the refractive index of the substrate (ng=1.445),xis the number of the high-index film layers in the reflective film stack (x=11),λ0is the central wavelength(λ0=532 nm), andmis the interference order of the filter (m= 1). At 532 nm wavelength,nHandnLare assumed to be 2.108 and 1.442[14]respectively for calculation, and the design value of half-power bandwidth is determined as 0.053 nm (53 pm). The bandwidth values calculated by Equation (1) are a series of separated values, among which 53 pm is the calculated value close to the target. Due to the influence of various technological factors in the actual film growth process and the existence of filmthickness monitoring error, the spectral passband will be widened to a certain extent.

Through “Film Wizard” film design software,the transmission spectrum curve of bandpass filter was obtained from the designed film system, as shown in Figure 1 (Color online). No antireflective film layer was designed on the other side of the substrate. When the light absorption of the film layer was not considered, the peak transmittanceTPreached 93% and the value of half-power bandwidth was 57 pm, it is little different from the result of Equation (1). When the light absorption of the film layer was considered and the extinction coefficients of the high and low index film layers (kH=1.5×10?5,kL=1.0×10?5) were introduced[15], the peak transmittanceTPdropped to 74% and the value of half-power bandwidth increased to 65 pm. In the band with wavelengths less than 531.5 nm and larger than 532.5 nm, another filter with a wider passband would be used for spectral interception.

2.2 Film fabrication

For ultra-narrow band-pass filters, their optical films are mainly prepared with two techniques: (1)Ion Beam Assisted Deposition (IBAD); (2) Dual Ion Beam Sputtering (DIBS). Comparatively speaking,the film prepared by DIBS technique is denser, and the film and its components are more reliable and stable[16-19]. In this paper, the DIBS technique is used to prepare thin films. The layout and working diagram of the vacuum chamber of the coating equipment are shown in reference[15]. The sputtering ion source RF16 is the main ion source, generating the converging high-energy (Ar+) ion beams to sputter the targets (Ta and SiO2targets). The (O2+and Ar+)mixed ion beams generated by RF12 are used to bombard the film in growth to achieve full oxidation and dense growth of the film. The purity of the targets is ≥99.99%. When the film was coated,the workpiece plate rotated at a high speed up to 800 r/min.

In the process of film deposition and growth,the Optical Monitoring System (OMS) emitted 532 nm light, which passed directly through a monitoring glass and was received by a detector. Then,an electrical signal was output from the detector to observe the change of light intensity. By substituting the filtering film system designed in Section 2.1 into the programming software “Film Maker”, the intensity change and trend of the transmitted light signal can be calculated and displayed, as shown in Figure 2. As can be seen from Figure 2, the optical monitoring signal passing from the 14thlayer to the 31stlayer tends to be flat with a small variation,which will bring a large monitoring error. This requires improving the monitoring method by introducing a new monitoring glass when starting the film coating in an insensitive layer. The “OptiLayer”film software is used to analyze the sensitivity of each film layer. The sensitivity represents the degree of the influence of film layer error on filter spectrum. The lower the sensitivity, the less the influence of the error. The sensitivity of each film layer is given in Fig. 3. It can be seen that the sensitivity increases rapidly from the 13th layer. Therefore,it is necessary to increase the variation amplitude of optical monitoring signal in the film layers 14?31 in order to improve the monitoring accuracy of the film layers. Therefore, after the deposition of the 12thlayer, a new monitoring glass should be used to monitor the deposition and growth of the next 32(13thto 44th) layers, thereby effectively improving the monitoring accuracy of the sensitive film layers.The Fig. 4 shows the change of optical monitoring signal with the increase of film layers after introducing two monitoring samples. It can be seen that the amplitude of variation is significantly increased.

Fig. 1 Transmittance spectra of the designed ultra-narrow band-pass filter圖1 設計的超窄帶濾光片透射光譜

Fig. 2 Variation of the transmittance of optical signal as a function of the number of film layers圖2 透射光控信號隨膜層的變化

Fig. 3 Sensitivity of film layer error on filter spectrum in each layer圖3 各個膜層誤差對光譜影響的敏感度

Fig. 4 Change of optical monitoring signal by using two separated monitor glass plates圖4 分成兩片監(jiān)控后光控信號的變化

As can be seen from Fig. 4(b), the monitoring signal in the 20thlayer on the monitoring glass 2 (the 32ndlayer as a whole) has hardly changed, so the time monitoring method is adopted for this layer.This is a layer of SiO2film. The average deposition time of the 3 layers of SiO2film in front of this layer is calculated by the control computer, as the deposition time of this layer. As can be seen from Fig. 3, the sensitivity of this layer is relatively low.The use of this method can achieve higher monitoring accuracy.

2.3 Spectral measurement

For an ultra-narrow band-pass filter with the bandwidth less than 1 nm, its transmission spectrum measurement requires more accurate instrument. The spectral resolution of the instrument should be better than 1/10 of the filter bandwidth. The spectral integral energy penetrating the filter is very small, so the instrument needs very high detection sensitivity and is required to effectively control electrical measurement noise and background light interference.

The spectral measurement setup is shown in Fig. 5. The light source is a supercontinuum laser source with a spectral range of 400?2000 nm, an output power of 8W, and a duty cycle of ≥99%.The light is led out of the optical fiber and is vertically incident on the surface of the filter after collimation. After being collected, the light penetrating the filter is led by the optical fiber into a spectral analyzer (Yokogawa AQ6373B). The wavelength range of 531.5?532.5 nm is selected, and the sampling interval is 0.003 nm. The measured transmittance spectrum curve of the filter samples is shown in Fig. 6. Some fluctuations can be seen on both sides of the passband. These fluctuations are measurement noises and the curve is not smoothed.

Fig. 5 Schematic diagram of the transmission spectrum measurement setup圖5 透射光譜測量裝置示意圖

Fig. 6 Measured transmission spectrum of the ultranarrow band-pass filter圖6 測量的超窄帶通濾光片的透射光譜

3 Analysis and discussion

3.1 Analysis of measurement spectrum

The comparison between Fig. 6 and Fig. 1 shows that the actually measured spectral passband is broadened and the peak transmittance is somewhat reduced. The specific data comparison is listed in Table 2.

Tab. 2 Data of measured and designed transmission spectrum表2 測量和設計的透射光譜數(shù)據(jù)

The film formed after actual deposition has a certain light absorption. In the process of deposition and growth, the optical control of film layer thickness always has some errors. Based on these two reasons, the actually measured spectrum is inconsistent with the designed spectrum.

(1) Effect of film absorption. The film absorption reduces the transmittance of optical energy, so that the transmittance of the spectral curve will decrease as a whole. As can be seen from Fig. 1 for this kind of ultra-narrow band filter, the peak transmittance is much smaller than that without film absorption, and the transmittance decreases a little at the wavelength far away from the peak wavelength.Thus the bandwidth corresponding to the half-peak value increases, and theBvalue is different from theAvalue, as shown in Table 2.

(2) Effect of film thickness control error. The control error of film thickness mainly includes the random error and the error caused by replacing the monitoring glass. The random error is usually very small, especially in the high-precision control system. Replacing the monitoring glass will change the initial growth state of the film, and thus will bring a certain error[20]. Due to the adoption of first-order vertical-transmission single-wavelength extremum monitoring method, the later deposited film layers can compensate for the error in the earlier film layers so that the optical thickness error of the whole film system will not be amplified. In order to investigate the influence of random error on the spectrum,the “OptiLayer” film software was used for simulation. An appropriate random error needs to be selected before simulation. Since the measured values of the filter bandwidth and central wavelength are not very different from the design values, the random error introduced should make the simulation results as close as possible to the design results, and should not be too large or too small. After several attempts,a random error of optical thickness equivalent to 10% of the passband width (i.e., 6 pm) was introduced to all layers. The simulation results are given in Fig. 7(a). Since the monitoring glass was replaced at the beginning of the 13th-layer plating, the effect of the random error of that layer on the spectrum needed to be investigated separately. For filters, the variations in passband width and central wavelength within 10% of the bandwidth are acceptable. After repeated attempts, the random error of each film layer was canceled, and a random error within 0.5% (i.e. 0.32 nm) of the optical thickness of the film layer was separately introduced into the 13thlayer to obtain the simulation results shown in Fig. 7(b). By using the DIBS deposition technique,the film thickness accuracy can be controlled to 0.5%.

Fig. 7 Variation of spectral curves after randomly introducing the control errors圖7 隨機引入控制誤差后的光譜曲線變化情況

As can be seen from Fig. 7, the introduction of these errors has little effect on the spectrum. The comparison with Fig. 6 indicates that the monitoring method used in this paper can control the random error within about 6 pm, thus forming a spectral passband with a width of 60 pm at 532 nm wavelength. The actual absorption of the film is larger than the set value, resulting in a further reduction of the peak transmittance. The bandwidth has no significant change, which is within the range of measurement error.

Fig. 8 Schematic diagram of the boundary between rough substrate surface and thin film圖8 粗糙基片表面與薄膜分界示意圖

3.2 Influence of substrate

The material and surface quality of the substrate have a certain influence on the spectral characteristics of this ultra-narrow band filter. The film deposition temperature is within the range of(100±5)℃, while the filter is usually used at normal temperature. Moreover, the environment may have temperature changes. Therefore, the linear expansion coefficientαof the film should be as close as possible to that of the substrate, and the refractiveindex temperature coefficients (dn/dT) of the film and substrate should be as small as possible. Alternative substrate materials are given in Table 3, including Crystal Quartz (CQ), several types of glass,and sapphire (Al2O3).

Tab. 3 Optical and thermal properties of substrates and thin films in this study[12, 21-22]表3 基片和薄膜的光學和熱特性[12, 21-22]

The CQ and sapphire (Al2O3) given in Table 3 have birefringence (but the data in Table 3 is for ordinary light (O light)) and a large linear expansion coefficient, so they were not selected in this study.Glass ceramics was also not selected due to its high refractive-index temperature coefficient. Optical glass (K9, BK7, etc.) is usually doped with some substances containing heavy metals, and exhibits a transmittance decrease under space irradiation,which has been confirmed in our previous experimental studies. Part of the data is from Schott's website. In this study, the JGS-1 fused quartz from China was selected as the substrate material.

The surface quality of the substrate also affects the spectral performance of the filter. If the surface is not smooth enough, a thin transition layer with an uncertain refractive index will be formed at the interface between the film and the substrate, as shown in Fig. 8, whereds-fis the geometric thickness of the transition layer. The refractive indexns-fof the transition layer is determined by Equation (2),

whereρsis the volume proportion of the substrate material in the transition layer,nsandnfare the refractive index of the substrate and that of the first layer respectively. The optical thickness deviation caused by the transition layer can be given by Equation (3),

It can be seen that a rougher surface has a larger Δdvalue accordingly.

In this study, Δdshould not be larger than 6 pm(10% of the bandwidth). Assuming the volume proportionρsof 0.5 and the first layer of Ta2O5and considering the film layer sensitivity shown in Fig. 4, the roughness P-V of the substrate surface should be less than 2 nm, so the surface is an ultrasmooth surface.

3.3 Spectrum stability

The spectral measurement of the filter samples was made one month after the completion of cutting. The samples with qualified spectra were selected as products and applied in the laser mapping system. After one month of aging, the stress in the film has been released, the film system tends to be stable. The optical stability of the spacer layer 2L has the greatest influence on the spectrum, while the influence of other layers is relatively small. The spacer layer is made of the same material as the substrate. Under temperature control, small temperature changes will not generate new stresses. The change of the optical properties of the film with temperature will lead to spectral shift, which can be approximated by Equation (4)[12]:

whereλ0is the preset central wavelength, andλTis the central wavelength at the temperatureT;Pis the number of cycles in the reflector film layers,P=11;Qis the order of the spacer layer,Q=1;is the optical thickness of the high-index layerHat the temperatureT, andis the optical thickness of the low-index layer L at the temperatureT.

By substituting the film system parameters of the filter into Equation (4), the change of its transmission spectrum with temperature was determined.The spectral drift is about 54 pm at the temperature change of 10℃, about 16 pm at the temperature change of 3℃, and about 2 pm at the temperature change of 2℃. In the actual optical system where the filter is applied, the temperature control condition is set to ±2℃. The filter behaves stably during the actual laser mapping process in space.

4 Conclusion

By using Ta2O5and SiO2as high and low refractive index film materials respectively and fused quartz as the substrate, we prepared an ultra-narrow band filter with a half-power bandwidth of (60±2)pm, a central wavelength of 532.0 nm and a peak transmittance of 62.6% through DIBS deposition.When the insensitive film layers were deposited, the monitoring glass was switched, the accumulation amplification of errors were effectively controlled by using two monitoring glass plates one after another and first-order transmission extremum monitoring method. Finally, an ultra-narrow band filter with a measured half-power bandwidth of about 60 pm was obtained.

——中文對照版——

1 引 言

波長為532 nm的綠色激光在大氣層中具有很好的穿透能力，相應的光源和光電接收器的性能穩(wěn)定。該波長的激光在激光雷達、自由空間光通信、空間激光遙感和三維測繪成像等方面有著良好的應用前景[1-4]。為了減少背景光干擾，特別是太陽輻射的強烈影響，需要利用通帶寬度在100 pm（即0.1 nm）以下的光譜濾波器來抑制背景光[5]。對于通帶寬度小于0.1 nm的光譜濾波要求，可采用的主要技術方式有：聲光調制技術、原子濾波技術、法布里-珀羅(F-P)標準具方式和薄膜干涉濾光技術等[6-8]。

對于空間光學儀器（特別是需要經(jīng)歷深空飛行的儀器），可靠性好、光學效率高和結構輕巧是需要重點考慮的因素，F(xiàn)-P標準具和薄膜干涉技術成為可選的主要技術方式。E. Troupaki[5]等用F-P標準具構建了光譜濾波組件，用于美國針對冰雪、云層和陸地進行高程測繪的ICESat-2衛(wèi)星上，實現(xiàn)了30 pm帶寬的光譜濾波，儀器用6束激光進行測繪。ICESat-2衛(wèi)星是目前國外唯一正在使用中的高程測繪衛(wèi)星。這種光譜濾波組件對溫度控制要求較高，在需要較多激光波束時難以合理布局眾多光譜濾波組件。為了發(fā)展更多激光波束的空間高程測繪，以精密光學薄膜為基礎，設計制作了目標帶寬為60 pm的帶通濾光片。

2 膜系設計與制備

幾種相關的亞納米光譜濾波技術各有特點（見表1），本文采用薄膜干涉光譜濾波技術。

2.1 濾光膜系設計

對于半功率帶寬非常窄的超窄帶濾光片，考慮各層薄膜在實際沉積時存在的誤差，采用單個諧振腔的F-P結構，形成全介質膜層的濾光片。其波形與采用F-P標準具得到的光譜波形相當，還可以用增加間隔層干涉級次的方法提升光譜的矩形程度。選用Ta2O5作為高折射率膜層，它在近紫外、可見光和短波紅外波段具有非常好的物理化學穩(wěn)定性和透明性[9-11]，適合制作超窄帶濾光片，如4G和5G光通信中使用的密集波分復用濾波器（DWDM）。本文選用SiO2作為低折射率膜層，SiO2具有良好的透明性和物理化學穩(wěn)定性，與基片熔融石英材質相同，有良好的熱匹配性。選用熔融石英作為濾光片基片，其熱穩(wěn)定性好，適合用作超窄帶濾光片的基片[12]，基片采用厚度為4 mm的JGS-1熔融石英。

濾光膜系設計為：Sub./ (1H 1L)101H 2L 1H(1L 1H)101L / Air。其中，H和L分別表示光學厚度為1/4中心波長的Ta2O5和SiO2膜層，中心波長就是532 nm激光波長。將低折射率膜層2L設計為間隔層，比起2H作為間隔層，它的線膨脹系數(shù)和折射率溫度系數(shù)更小一些，濾光片的光譜也會更穩(wěn)定一些。當然，這種膜系對入射光線的角度也會敏感一些。這是單個諧振腔的F-P結構，其完全由介質薄膜構成。

通帶寬度計算，可以由公式(1)得到近似值[13-14]，

其中，nH和nL分別為高低折射率膜層的折射率，ng為基片的折射率（取值1.445），x為反射膜堆內高折射率膜層數(shù)（取值11），λ0為中心波長（取值532 nm），m為濾光片的干涉級次（m= 1）。在波長532 nm處，nH和nL分別取為2.108和1.442[14]進行計算，得到半功率帶寬的設計值為0.053 nm(即53 pm)。利用公式（1）計算得到的帶寬值是一系列分離的數(shù)值，53 pm是接近目標的計算值。由于在實際薄膜生長的工藝過程中存在多種工藝因素的影響，還存在膜層的厚度監(jiān)控誤差，光譜通帶會有一定程度的展寬。

將設計膜系代入Film Wizard光學薄膜設計軟件，得到帶通濾光片的透射光譜曲線，在圖1（彩圖見期刊電子版）中給出，在基片另一面沒有設計減反射膜層。在不考慮膜層光吸收的情況下，峰值透過率Tp達到93%，半功率帶寬的數(shù)值為57 pm，與式（1）差異不大；在考慮膜層光吸收的情況下，引入高低折射率膜層消光系數(shù)[15]：kH=1.5×10?5和kL= 1.0×10?5，峰值透過率Tp降低到74%，半功率帶寬的數(shù)值增加到65 pm。在波長小于531.5 nm和大于532.5 nm的區(qū)域，將由另外的通帶較寬的濾光片進行光譜攔截。

2.2 薄膜制備

對于超窄帶濾光片，光學薄膜的制備技術主要有兩種：（1）離子束輔助沉積技術（Ion Beam Assisted Deposition，IBAD）；（2）雙離子束濺射技術（Dual Ion Beam Sputtering，DIBS）。相對而言，DIBS技術制備的薄膜更加密實，薄膜和元件的可靠性和穩(wěn)定性也更好[16-19]。本文采用DIBS技術制備薄膜，鍍膜設備真空室內的布局和工作示意圖見文獻[15]。其中濺射離子源RF16為主離子源，它產生的高能會聚（Ar+）離子束用于濺射靶材料（Ta和SiO2靶）；輔助轟擊離子源RF12產生的（O2+和Ar+）混合離子束對生長中的膜層進行轟擊，實現(xiàn)膜層的充分氧化和致密生長。靶材的純度≥99.99%，鍍膜時工件盤高速轉動，轉速達到800 r/min。

在薄膜沉積生長過程中，光學監(jiān)控系統(tǒng)（OMS）發(fā)出532 nm波長的光直接穿過監(jiān)控片，被探測器接收后輸出電信號，用于觀測光強度的變化。將前面2.1節(jié)中設計的濾光膜系代入編制軟件Film Maker中，可以計算并顯示出透射光信號的強弱變化和走勢，如圖2所示。從圖2可以看出，從第14層到第31層光學監(jiān)控的信號趨于平坦，變化幅度很小，這樣會給監(jiān)控帶來較大誤差。因此，需要改進監(jiān)控方式，在一個不敏感層開始鍍膜時切入一個新的監(jiān)控片。用OptiLayer光學薄膜軟件分析了各個膜層的敏感度，敏感度表示該膜層誤差對濾光片光譜的影響程度，敏感度越低則誤差的影響越小。圖3中給出了各個膜層的敏感度，可以看出從第13層開始敏感度迅速增大。所以，必須提高第14到31層薄膜光控信號的變化幅度，才能提升膜層的監(jiān)控精度。因此，在第12層薄膜沉積完成后，更換一個新的監(jiān)控片來監(jiān)控后面32層（第13到44層）薄膜的沉積生長，以有效提升敏感膜層的監(jiān)控精度。圖4給出了采用兩個監(jiān)控樣品后，光學監(jiān)控信號隨膜層的變化情況，可見變化幅度明顯提升。

從圖4（b）可以看到，監(jiān)控片2上的第20層（總第32層）的監(jiān)控信號幾乎沒有變化，對該層采用時間監(jiān)控方法。這是一層SiO2薄膜，由控制計算機計算出它前面3層SiO2薄膜沉積的平均時間，作為該層薄膜的沉積時間。從圖3可以看到，該層薄膜的敏感度是比較低的。采用這樣的方法總體可以得到更高的監(jiān)控精度。

2.3 光譜測量

帶寬小于1 nm的超窄帶濾光片的透射光譜測量需要更加精確的光譜測量儀器。儀器的光譜分辨能力應該優(yōu)于濾光片帶寬的1/10；能夠透過濾光片的光譜積分能量很小，儀器需要很高的探測靈敏度，同時能夠有效控制測量電噪聲和背景光干擾。

光譜測量裝置如圖5所示。光源選用超連續(xù)譜的激光光源，光譜范圍為400～2000 nm，輸出功率為8 W，占空比≥99%；光線由光纖導出，準直后垂直入射濾光片表面；透過濾光片的光線被收集后由光纖導入光譜分析儀器（日本橫河公司生產的AQ6373B光譜分析儀），選定531.5～532.5 nm波長，采樣間隔為0.003 nm。圖6是測量得到的濾光片樣品的透射率光譜曲線，在通帶的兩側看到一些波動，這些波動是測量噪聲，曲線沒有經(jīng)過平滑處理。

3 分析和討論

3.1 測量光譜分析

對比圖6和圖1可以看出，實際測得的光譜通帶有所展寬，峰值透過率也有所降低，具體數(shù)據(jù)對比見表2。

實際沉積制成的薄膜對光有一定的吸收；在沉積生長過程中，膜層的光學厚度控制也總是存在一些誤差?；谶@兩方面原因，造成了實際測量光譜與設計光譜不一致。

（1）膜層吸收的影響。膜層的吸收會減少透過的光學能量，使得光譜曲線的透過率整體下降。從圖1可以看出，對于這種超窄帶濾光片，峰值透過率下降較多，離峰值波長較遠處下降較少，這就使得半峰值對應的帶寬增大，出現(xiàn)了表2中B值和A值的差別。

（2）膜厚控制誤差的影響。膜層厚度的控制誤差主要來自隨機誤差和更換監(jiān)控片帶來的誤差，隨機誤差通常很小，特別在高精度控制系統(tǒng)中，更換監(jiān)控片改變了薄膜的初始生長狀態(tài)，會帶來一定的誤差[20]。由于采用了1級次的垂直透射式單波長極值監(jiān)控方法，后續(xù)沉積的膜層對前面制成的膜層有誤差補償作用，可以使得整個膜系的光學厚度誤差不被放大。

為了考察隨機誤差對光譜的影響，用OptiLayer光學薄膜軟件進行模擬。在進行模擬之前需要選擇合適的隨機誤差。由于濾光片的帶寬和中心波長的實測值與設計值差距不大，因此引入的隨機誤差應使模擬結果盡量接近設計結果，不宜過大或過小。在進行多次嘗試后，引入了光學厚度相當于通帶寬度10%（即6 pm）的隨機誤差到所有膜層，并給出了模擬結果（圖7（a））。由于在鍍制第13層薄膜開始時更換了監(jiān)控片，所以需要單獨考察該層的隨機誤差對光譜的影響。對于濾光片而言，通帶寬度和中心波長的變化在帶寬的10%以內是可以接受的。同樣進行多次嘗試后，取消各個膜層的隨機誤差，再單獨引入0.5%光學厚度（即0.32 nm）以內的隨機誤差到第13層薄膜中，并給出了模擬結果（圖7（b））。對于用雙離子束濺射薄膜沉積技術而言，膜厚精度控制到0.5%是可以實現(xiàn)的。

從圖7可以看出，這些誤差的引入對光譜的影響不大。對比圖6可知，本文所用的監(jiān)控方法可以把隨機誤差控制在約6 pm以內，因此能在532 nm波長處形成寬度為60 pm的光譜通帶。薄膜的實際吸收能量比設定值要大一些，導致峰值透過率進一步降低，而帶寬沒有明顯變化，變化量在測量誤差范圍內。

3.2 基片的影響

基片材質和表面質量對超窄帶濾光片的光譜特性有一定的影響。薄膜沉積溫度在（100±5）℃范圍，濾光片通常在常溫環(huán)境中使用，考慮到使用環(huán)境可能的溫度變化，薄膜和基片的線膨脹系數(shù)應盡可能接近，薄膜和基片材料的折射率溫度系數(shù)dn/dT應盡量小?？晒┻x擇的基片材料在表3中給出，這些材料是石英晶體（Crystal Quartz,shoux, CQ）、幾種玻璃和藍寶石（Al2O3）。

表3中，石英晶體（CQ）和藍寶石（Al2O3）有雙折射現(xiàn)象，表3中數(shù)據(jù)選用的是尋常光（O光）的數(shù)據(jù)，它們的線膨脹系數(shù)也較大，故在此研究中沒有選用。微晶玻璃的折射率溫度系數(shù)較高，也沒有選用，光學玻璃（K9、BK7等）中通常摻雜了一些含有重金屬的物質，空間輻照條件下其透過率會下降，本課題組以前的研究已經(jīng)證實。部分數(shù)據(jù)來源于德國Schott公司網(wǎng)頁。本研究選擇中國牌號的JGS-1熔融石英作為基片材料。

基片的表面質量也會影響濾光片的光譜性能。不夠光滑的表面，在薄膜與基片的界面處會形成一層折射率不確定的過渡薄層，如圖8所示。其中ds-f是過渡層的幾何厚度。這個過渡層的折射率ns-f由式（2）決定：

其中ρs是基片材料在過渡層的體積占比，ns和nf分別是基片的折射率和第1層薄膜的折射率。由過渡層引起的光學厚度偏差可以由式（3）給出：

可以看出，粗糙表面對應的Δd值也較大。在本研究工作中，Δd應該不大于6 pm（帶寬的10%），按照ρs取0.5和第1層用Ta2O5薄膜來計算，結合圖3考慮到膜層的敏感度，基片表面的粗糙度P-V值應該小于2 nm，屬于超光滑表面。

3.3 光譜的穩(wěn)定性

濾光片樣品的光譜測量是在切割完成一個月后進行的，篩選出光譜合格的樣品作為產品應用在激光測繪系統(tǒng)中。經(jīng)過一個月的老化，薄膜中的應力已經(jīng)釋放，薄膜體系趨于穩(wěn)定。對光譜影響最大的是間隔層2L的光學穩(wěn)定性，其他膜層的影響相對小一些。間隔層與基片的材質相同，在溫控條件下，小的溫度變化不會產生新的應力。膜層光學特性隨溫度的變化會導致光譜的移動。光譜的移動量近似得[12]：

其中λ0是設定中心波長，λT是T溫度下的中心波長；P是反射板膜層的周期數(shù)，這里取11；Q是間隔層的級次，這里取1；是T溫度下高折射率膜層H層的光學厚度，是T溫度下低折射率膜層L層的光學厚度。

將濾光片膜系參數(shù)帶入公式（4），計算得到其透射光譜隨溫度變化的情況，即溫度變化10 ℃時光譜漂移約54 pm，溫度變化3 ℃時光譜漂移約為16 pm，溫度變化2 ℃時光譜漂移約為2 pm。濾光片的實際應用光學系統(tǒng)，溫控條件設置為±2℃，濾光片在空間實際激光測繪過程中表現(xiàn)穩(wěn)定。

4 結 論

采用Ta2O5和SiO2分別作為高低折射率膜層材料，熔石英為基片，利用雙離子束濺射沉積方法制備了半功率帶寬為（60±2） pm超窄帶濾光片，其中心波長為532.0 nm，峰值透過率達到62.6%。在不敏感膜層沉積時切換監(jiān)控片，依次用2個監(jiān)控片和1級次透射極值監(jiān)控的方法，有效控制了誤差的影響和積累放大，實測得到半功率帶寬約為60 pm的超窄帶濾光片。