ОПТИКА И СПЕКТРОСКОПИЯ, 2014, том 117, № 3, с. 496-501
ФИЗИЧЕСКАЯ ^^^^^^^^^^^^^^^^ ОПТИКА
УДК 681.7.068
COMBINED OPTICAL DISPERSION BY PRISM AND ARRAYED WAVEGUIDE GRATING WITH MULTIPLE DIFFRACTION ORDERS FOR RAMAN SPECTROMETER
© 2014 г. Ya-qin Cheng*, Hong-da Sun*, Zhao Wu*, Sheng-feng Deng**, Miao Lu**
*MEMSResearch Center, Xiamen University, Xiamen, China **Pen-Tung Sah Micro-Nano Technology Institute, Xiamen University, Xiamen 361005, China
E-mail: lm@xmu.edu.cn Received January 25, 2013
A compact dispersive device for Raman spectrometer was proposed to achieve a spectrum resolution below 0.55 nm in the spectral range of 800 to 1000 nm. A 41-channel arrayed waveguide grating (AWG) with eleven different diffraction orders was designed, and each output channel of this AWG contained eleven light signals with periodically 20 nm spaced wavelength. These signals were further cross-dispersed by a prism, and finally form a 41 x 11 spots array on a CCD. The detailed theoretical analysis and simulation of this dispersive device were introduced in this paper. Compared with commercial dispersive modules composed of grating, lens and mirrors, the proposed structure is able to provide a compact device with higher spectrum resolution, which is attractive for handheld Raman spectrometer.
DOI: 10.7868/S0030403414090281
INTRODUCTION
Raman spectroscopy is able to detect chemical or biological molecules even with H2O or CO2 existence, and co-existing chemicals can be identified simultaneously according to their unique characteristic spectral peaks. So Raman spectroscopy is believed to be a kind of simple, speedy and powerful detection tool to identify chemicals in environment, food and drugs. However, the applications of Raman spectroscopy nowadays are severally limited by the expensive and bulky spectrometer. Theoretically, the spectral resolution of a traditional spectrometer composed of lens, mirrors and grating will drop significantly with its size scaled down. So some new architecture must be employed for low coat and compact Raman spectrometer. Meanwhile, arrayed waveguide grating (AWG) has been regarded as a substitute of bulky dispersive system, and was used successfully as a wavelength demultiplexer in optical communication system. AWG is prepared by micro fabrication technology and realizes the optical dispersive function on a chip, so a low-cost, handheld Raman spectrometer can probably be developed based on this compact dispersive chip.
Some AWG designs had been reported for spectrometer applications. Y. Komai et al. reported a compact spectroscopic sensor using a visible range 8 channel AWG [1]. K. Kodate et al. developed a hybrid chip integrating AWG device, laser diode and detector array to detect the concentration of chlorophylls in water, and an 8 channel AWG with 12.5 nm spectral resolution and 800 nm central wavelength was demonstrated with less than 4 dB insert loss [2]. P. Cheden et al. pre-
pared an 8 x 8 mm2, 50 channel AWG dispersive chip on SOI substrate, and its spectral resolution reached 0.2 nm at a central wavelength about 1545 nm [3]. N. Ismail et al. reported a 55 channel AWG with 800— 920 nm spectral range on silicon-oxynitride waveguide to in-vivo detect the water concentration in human's skin [4]. N. Cvetojevic et al. utilized an AWG for astronomy to observe atmospheric molecular OH emission lines, and the demonstrated AWG had a free spectral range (FSR) of 57.4 ± 0.6 nm and a spectral resolution of 0.75 nm [5]. V.D. Nguyen et al. designed and fabricated an AWG based Optical Coherence Tomography, and the AWG has a footprint of 3.0 cm x 2.5 cm, operates at a center wavelength of 1300 nm, and has a FSR of 78 nm [6]. L. Chang et al. investigated another optical coherence tomography with an AWG center on 800 nm and a FSR of 19.4 nm [7]. Recentli S. Mu-rugkar et al. developed an on-chip miniature AWG spectrometer based on slow light to realize high spectral resolution [8]. N.A. Yebo et al. integrated a 16 channel, 200 GHz (1.6 nm) space AWG with a micro-ring resonator for gas sensing [9, 10]. AWG with asymmetric structure was proposed by M. Pollnau's group in order to obtain low phase error and low insert loss [11—14]. However, all these reported AWG devices didn't exhibit a competitive performance with commercial portable Raman spectrometers, mainly due to high resolution and broad spectral range came at the cost of high insert loss and large device size for AWG.
Spectral range of a Raman spectrometer normally covers several hundred nanometers, so hundreds of output channels were required to get a typical spectral resolution below 1 nm. A low diffraction order m and
a reduced increment (AL) of the arrayed waveguide below 10 ^m are unavoidable, and the layout of such an AWG is hard to draw using widely used commercial AWG design tools because of the overlapping of consecutive waveguides. Even if the layout can be constructed, its footprint will be too large to be fabricated in a wafer with an acceptable yield.
Cascaded AWG configurations were proposed in recent literatures to realize broadband, high-resolution spectrometers. K. Takada et al. reported a 1010-channel cascaded AWG filter with 10 GHz channel spacing, wherein a 10-channel AWG acted as the pre-stage to assign lights into ten 160-channel second-level AWGs, but the insert loss was up to 13 to 19 dB [15]. B. Mo-meni et al. proposed a cascaded structure consisting of a low resolution and wideband AWG as the first stage, followed by high resolution super-prism devices based on photonic crystals [16], and this kind of super-prism was discussed in details in several other papers published by the same group [17—19]. B. Momeni et al. also demonstrated another cascaded configuration including an AWG followed by a set of resonators, experimental results showed the capability of such spectrometer to achieve ~0.1 nm wavelength resolution over 10 nm bandwidth in a compact device in millimeter scale [20]. However, a cascaded AWG system normally had an insert loss more than 10 dB and a non-uniform passband, which hindered its application in Raman spectrometer. In recent, B.I. Akca et al. presented a new synchronized design for flattening the passband of a cascaded AWG system, which consisted of the low-loss flat-top AWG as a primary filter and five 1 x 51 AWGs with a 0.4-nm channel spacing as secondary filters, but the overall insert loss was still high [21].
Another approach to realize broadband and high spectral resolution simultaneously is to adopt multiple diffraction orders. In this case, multiple overlapping lights with periodically spaced wavelength were contained in each output waveguide. N. Ismail et al. prepared an AWG with multi-order overlapped spectrum to test human's teeth containing localized initial carious lesions, and achieved a spectral range of 859—957 nm and a resolution of 0.2 nm [22]. Unfortunately, these multiple lights in an output waveguide must be further dispersed in most spectrometer applications. J. Bland-Hawthorn et al. developed an integrated cross-dispersive optics to avoid overlapping of lights from the same output channel, and succeed to identify the optical signals from an astronomical telescope in a wide spectral range. However, the cross-disperser was not discussed in details in the papers [23, 24]. In an illustration figure, the lights are focused by a micro cylinder lens before being cross-dispersed by a micro prism, but lateral overlapping of lights from adjacent output channels will probably occur.
In this paper, a new cross-disperser for a multi-order AWG was proposed for broadband Raman spec-
^--Paraboloid-shaped micro mirror array
Fig. 1. The illustration of the structure of the proposed dispersive device.
trometer. This cross-disperser consisted of a parabolic-shaped micro mirror array and a prism, this micro mirror array was fabricated on the same chip with AWG, and the prism was glued on the chip by optical adhesive. As a demonstration of this concept, a dispersive device with 800—1000 nm range and about 0.55 nm spectral resolution was designed and simulated, it can be realized in a volume of several cubic centimeters and is promising for handheld Raman spectrometer.
CONCEPT OF THE DISPERSIVE DEVICE
As shown in Fig. 1, the proposed dispersive device consists of three components: an AWG chip, a prism and a parabolic-shape micro mirror array. The AWG in this configuration worked on multiple diffraction orders, and contained multiple lights with periodically spaced wavelength in each output waveguide. A parabolic-shape mirror array was built, and the end of each output waveguide of the AWG located exactly at the focal point of the parabolic-shape mirror. Therefore, the light from each output waveguide was reflected upwards and collimated. A prism with high index of refraction (PS851, Thorlabs Co., USA) was glued on the AWG wafer using a kind of optical adhesive (NOA13685, Norland Co., USA) in the way that one of its surface was set close to the ends of output waveguide of AWG. In this configuration, the lights in the same output waveguide were cross-dispersed by this prism.
DESIGN AND SIMULATION OF AWG
WITH MULTIPLE DIFFRACTION ORDER
AWG Based on Silicon Oxynitride Waveguide
An AWG with multiple diffraction orders was designed based on silicon oxynitride waveguide. Through changing the flow rate of SiH4 and N2O in the deposition process, silicon oxynitride with refractive index of 1.483 and 1.468 was prepared to act as the core and cladding layer, respectively. The layout of the AWG was
498
YA-QIN CHENG et al.
-i 0.56
Fig. 2. The layout of the AWG with eleven diffraction orders based on silicon oxynitride waveguide.
shown in Fig. 2, here the footprint of the 41 channel AWG is 23 mm x 14 mm and the cross-section of waveguide is about 2 ^m x 1.5 ^m.
The Determination of Diffraction Order
Just like other kinds ofgrating, an AWG also has the spectral periodicity. When a beam wit
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.