A new compact infrared spectrometer without any mechanical moving elements has been designed and constructed using a two-dimensional InGaAs array detector and 10 sub-gratings. The instrument is compact, with a double-folded optical path configuration. The spectra are densely 10-folded to achieve 0.07-nm spectral resolution and a 2-ms data acquisition time in the 1450- to 1650-nm wavelength region, making the instrument useful for real-time spectroscopic data analyses in optical communication and many other fields.
©2009 Optical Society of America
In the modern information age, compact spectrometers without any mechanical moving parts are needed for a wide variety of applications . In cases such as the optical communication field, where in situ spectral data analysis is critical, it is important to measure and analyze the spectral data at very high speed with high spectral resolution and reliability . It is difficult for traditional optical designs to fulfill all three key functions (i.e., wide working wavelength range, fine spectral resolution and high speed) in a single spectrometer due to limitations on the optical features of the photonic elements used in the instrument. In the optical communication field, for example, more than 2000 data points will be required to measure the spectral signal in the 1450- to 1650-nm wavelength region with a wavelength resolution narrower than 0.1 nm. Until now, there has been a hard optical barrier limiting infrared array detector pixel density. Most spectrometer designs in both the infrared and visible regions of the spectrum use a conventional mechanical approach to scan and rotate the dispersive optical elements (like the prism and grating) in very fine steps to achieve high spectral resolution [3–7]. However, this approach consumes time during the data scanning process and negatively affects the repeatability of the data. Other fields would also benefit from faster, more reproducible IR spectrometers. In film growth, for example, the spectral characteristics of the advanced optical device need to be measured in situ. Spectrometers with mechanical moving parts encounter difficulty in this type of application .
Advanced optical designs in which multiple sub-gratings are applied and integrated with multiple one-dimensional InGaAs array detectors in the 1450- to 1650-nm infrared region allow both high-speed data readout and fine spectral resolution .
This work builds on earlier efforts to improve the performance of the spectrometer in the infrared region. Specifically, we have used a new, compact design in which a two-dimensional InGaAs array detector is integrated with 10 sub-gratings in a spectrometer with a double-folded optical path configuration. The spectra have been densely 10-folded to achieve 0.07-nm spectral resolution and a 2-ms data acquisition time in the 1450- to 1650-nm wavelength region.
2. Principle and system construction
The 10 sub-gratings used in the design are plane reflection gratings blazed at 1500 nm, with a groove density of 600 g/mm. As indicated in Fig. 1, light with wavelength λ incident at angle i onto the grating will be diffracted at angle θ, which is located on the same side of the normal direction n̂ of the grating surface according to the diffraction equation:
where k is the diffraction order. The diffraction order in this particular application is equal to 1 because only the first order of the diffraction light is considered in this work. The grating constant d in Eq. (1) is 1.67 µm for a grating with the groove density chosen in the design.
The spectrometer configuration is shown in Fig. 2. The spectrometer has been designed with a focal length of 700 mm. Light enters through the entrance slit, which is the exit of a single-mode fiber with a diameter of ~10 µm. To make the spectrometer compact, the plane mirror M1 is used to fold the light path, directing the light to a spherical mirror M2 that reflects and collimates the light incident onto grating G.
As seen in Fig. 3, the integrated grating G consists of 10 sub-gratings. Each of these gratings is set at a slightly different angle with respect to each sub-wavelength region as calculated according to Eq. (1). By matching to the 1450- to 1650-nm spectral region used in the optical communication field, 10 sub-wavelength regions are uniformly distributed with a wavelength window of about 20 nm for each sub-grating. The gratings were physically arranged (from bottom to top) such that they allowed collection of the following spectral windows: 1450–1470 nm, 1470–1490 nm, 1490–1510 nm, 1510–1530 nm, 1530–1550 nm, 1550–1570 nm, 1570–1590 nm, 1590–1610 nm, 1610–1630 nm, and 1630–1650 nm. Each sub-grating is 100 mm×10 mm, with a groove density of 600 g/mm . The spectral resolving power R in the first order is defined as R=λ/Δλ=N, where N is the total number of grooves along the spectral direction on the grating . For a grating with a size of 100mm along the spectral direction and a groove density of 600 g/mm, N=6×104 and the wavelength resolution Δλ will be equal to about 0.025 nm at a typical wavelength of 1500 nm. By combining 10 sub-gratings, therefore, the integrated gating G has a size of about 100 mm×100 mm. A mask with a long, rectangular window is placed on the surface of each sub-grating by measuring and checking the neighboring pixel signals to make sure there will be no possible diffraction and stray light cross-talk between neighboring sub-gratings.
The spectrometer is designed to have spectral dispersion of about 2.3 nm/mm at the focal plane of the detector, which will then be required to have a photon sensing size of at least 87 mm along the spectral direction in the 1450- to 1650-nm wavelength region. There is a lack of one-dimensional infrared array detectors with such a physical size in the optical communication field at the moment. Therefore, a two-dimensional InGaAs infrared array detector has been used in this work . The detector has an effective photon sensing area of 9.6 mm(x)×7.68 mm(z) (320×256 pixels), with a pixel size of 30 µm×30 µm and 12-bit A/D signal converting capability. According to the dispersion features of the spectrometer, each pixel will have wavelength resolution of about 0.069 nm.
The focal plane of the array detector can be divided into 10 sub-areas along the z direction. Each sub-area has a size of about 9.6 mm(x)×0.77 mm(z) (320×26 pixels). These 10 sub-areas on the focal plane of the array detector can be re-arranged by software to correspond to the areas illuminated by the 10 sub-gratings working in the 1450- to 1650-nm wavelength region. The effective photon sensing area required to cover the entire spectral window of 200nm along the spectral (x) direction is 96 mm(x)×0.77 mm(z) (about 3200×26 pixels). As a result, there will be a region about 9 mm in size (0.9 mm for each sub-area) left along the x direction on the focal plane of the detector for adjustment of the position of each sub-grating during the data reduction process.
As seen in Fig. 2, two cylindrical mirrors M3 and M4 with different focal lengths are used in the design. M3 has a focal length of 700 mm, with its cylindrical axis aligned along the z direction to focus the diffracted light on focal plane P of detector D along the spectral (x) direction. M4 has a focal length of 375 mm and is aligned with its cylindrical axis along the x direction to fold the light path and direct the diffracted light from the 10 sub-gratings onto the focal plane of the detector along the z direction.
3. Results and discussion
An Ultra-Wideband Source (UWS-1000)  emitting in the infrared optical communication region of the spectrum has been used to align and tune each of the optical elements, by monitoring the spectral image on the focal plane of the two-dimensional InGaAs array detector. As seen in Fig. 4, the 10 wavelength windows are clearly seen and nearly uniformly distributed in space from 1450 to 1650 nm, with a spectral window of about 20 nm.
A tunable laser (MLS-8100) emitting at optical communication wavelengths  was used to calibrate the spectrometer. The results are shown in Fig. 5. As mentioned above, the total pixel number has been extended to 3200 in the spectral direction along the x axis by densely folding the spectral image. About 2900 pixels are required to cover the 200-nm spectral region in the design. The remainder of the pixels can be used for calibration purposes. As a result, two neighboring wavelength windows can be seamlessly connected by software in the data reduction procedure. Each 9.6-mm sub-window along the x axis of the spectral direction at the focal plane of the array detector shows a nearly linear relationship between wavelength and pixel number. The average spectral resolution is ~0.068 nm, which agrees well with our initial expectations based on the system design.
For testing the utility of the spectrometer for use in the optical communication field, a commercialized the output of a dense-wavelength-division-and-multiplication (DWDM) device with 4 transmission channels has been measured using the Ultra-Wideband Source (UWS-1000). Each channel of the DWDM device is spectrally separated by ~0.8 nm, with peak centers located at 1550.12 nm, 1550.92 nm, 1501.72 nm and 1502.52 nm. The measured transmission spectrum of the DWDM device with a data acquisition time of about 2 ms is shown in Fig. 6. The measured wavelength separation between two neighboring channels is about 11–12 pixels on average, corresponding to resolution of 0.067–0.072 nm/pixel. This value is in good agreement with the system design.
Another commercialized coarse-wavelength-division-and-multiplication (CWDM) device with 4 transmission channels has also been measured. Each channel of the CWDM device has wavelength separation of about 20 nm, with the output centered at 1470 nm, 1490 nm, 1510 nm and 1530 nm. The measured transmission spectrum of the CWDM device with a data acquisition time of about 2 ms is shown in Fig. 7. The spectral pattern for each channel can be quickly measured and analyzed with high precision. The results show that the spectrometer is efficient and reliable and that it is suitable for use in optical communication and related fields.
Another application for the spectrometer is the in situ spectral monitoring of thin-film devices grown under vacuum. A schematic diagram of the electron beam evaporation system integrated with the broadband spectrometer is shown in Fig. 8. The probe beam of a bright light source (Santec, UWS-1000)  emitting in the 1200–2000 nm wavelength range was focused to a beam size of ~3 mm to examine the sample in situ. The transmitted light beam exiting the vacuum chamber was coupled into an optical fiber which was connected to the entry slit of the spectrometer. The transmittance spectrum of the sample grown in the chamber was measured in situ with high speed and high spectral resolution. The spectral features of the multi-layered thin film structure were analyzed in terms of spectral parameters, such as the amplitude, bandwidth and center wavelength, and compared with the pre-designed and calculated data to precisely control the film growth process.
The multi-layered sample consisted of SiO2 and Ta2O5 layers deposited on a Si substrate that had been polished on both sides. The best fitted refractive index of SiO2, Ta2O5 and Si at the designed wavelength of 1540 nm is 1.466, 1.955 and 3.47, respectively. The sample produced in this demonstration has 27 layers, with a film structure of Si/9L/[1H1L]4/2H/[1L1H]4/air. L and H indicate the low and high refractive index film materials (SiO2 and Ta2O5, respectively). The optical layer thickness is equal to one quarter of the designed wavelength. Transmittance spectra of the sample with 25, 26 and 27 layers are shown in Fig. 9(a). These measured spectra agree well with the calculated data over the relevant spectral range. The central wavelength is exactly located at the designed value of 1540 nm.
Both the measured and calculated transmittance spectra of the sample with a fractional number of the layer thickness are in good agreement with the results shown in Fig. 9(b). The peak wavelength of the spectrum is shifted to ~1542.7 nm and ~1535.7 nm for fractional layer numbers of 25.2 (0.2 SiO2 layer) and 26.7 (0.7 Ta2O5 layer), respectively. The shift of the peak wavelength with changes in the spectral pattern of the fractional layer structure cannot be dynamically monitored efficiently by an optical method using a fixed wavelength. These spectra can be precisely measured and analyzed by the broadband spectrometer which was designed and constructed in this work to achieve high spectral resolution with data acquisition speed able to monitor film growth in situ at a rate of about 0.5–1.0nm/s. Therefore, the spectrometer may find significant use in applications such as in the in situ monitoring and control of film growth to yield structures which contain either integer or non-integer numbers of the layer thickness.
A new, compact infrared spectrometer without any mechanical moving elements has been designed and constructed using a two-dimensional InGaAs array detector and 10 sub-gratings. This compact instrument uses a double-folded optical path configuration. Spectra in the 1450- to 1650-nm wavelength region are densely 10-folded to achieve 0.07-nm spectral resolution and 2ms data acquisition time. The resolution and acquisition speed are about 11% and 20-fold better, respectively, than the results achieved using one-dimensional array detectors . The spectrometer has been tested to measure the spectrum of commercial DWDM and CWDM devices, and has been applied to in situ measurement and analysis of the transmittance spectra of vacuum deposition samples with either integer or fractional numbers of layer thicknesses. The results show that the instrument is suitable for real-time spectroscopic data measurement and analysis, and that the precision is sufficient for use in optical communication and many other fields.
This work was supported by contract number #60778028 from the Chinese National Science Foundation (CNSF), and by the Chinese STCSM project (Grant No. 07TC14058).
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