1.1

Machined grating

In the outstanding diffraction grating, since the interval accuracy (RMS) which is less than 1/100 of wavelengths is required, the mechanical diffraction grating has been manufactured by the ruling machine, which specialized in this accuracy conventionally. However, high-precision vibration, temperature, and structural analysis became possible by improvement in simulation technology, and machine tool accuracy improved remarkably. These have the positioning accuracy of Nano-order, and, originally can process a diffraction grating with sufficient accuracy in spite of general-purpose cutting or grinding processing equipment. Since a specular surface is enough obtained only by processing, if high-end cutting in recent years and a grinding processing machine have grid spacing accuracy and enough scale stability, they have the potential of diffraction grating processing. In Canon, the machine for in-company processing has developed according to application. The highly precise surface is especially required of metallic mold processing for optical components, and “A-Former” shown in Fig. 1 is satisfied with these demands.

Figure 1.

The free-form processing machine (A-Former).

The diffraction grating, which is kind of an echelle grating, is required for sharp edge and plane, which have the outstanding linearity. Moreover, as for the angle of the edge of two plane, nearly 90 degrees is best in general. Since a shape of tool is transferred as form (shape) by cutting, “cutting” is very suitable the processing method for this geometry demand. Since the single crystal diamond tool has the outstanding linearity and very small edge, it is equipped with the potential, which processes an ideal diffraction grating in Figure 2.

Figure 2.

The typical cross section of groove by cutting

1.2

Immersion grating

Although an immersion grating is one of the classic grating¹, it is recently that acquisition generally became possible as an object for infrared region except Si and ZnSe. This is that grooved processing to a brittle material has achieved with improvement in cutting accuracy, and it originates in grooved processing to a semiconducting material with the transmittance characteristic excellent in the infrared region having become easy. In Canon, the immersion grating by the materials, which can cover an infrared region widely and has a large refractive index, has been developed. Now, the materials currently provided is shown in table 1. However, as this paper shows, there is also a still incomplete domain about coating and machined processing required for actual use, and there are many points to be developed continuously.

Table 1.

Material characters for the immersion device, which Canon provides in 2018

2.1

CdZnTe

It is well known that CdZnTe is the material widely used as a substrate of an infrared detector (HgCdTe imager arrays). Since the character of mechanical machining is good, it is also the first immersion grating, which we shaped by cutting in 2012². Machining examination of grooving, which the size of the device and grooved pitch differ also after this report had advanced, and the large-sized with 80mm in length grating had be succeeded in 2013³. However, the stable manufacture processes, such as the reproducibility of machining and coating, had not established yet.

2.2

Machining

In the process of practicality examination, since the process of the grooving by 2013 was unstable, the process was changed radically in 2016. It is necessary to control load in the process of grooving for a brittle material like a CdZnTe to the occurrence of a chip does not produce in the ridgeline. Figure.3 shows the CdZnTe immersion grating by latest machining process with the pitch of 82μm, the parameters of the grating and the picture of grooves by a scanning electron microscope (SEM).

Figure 3.

The machined immersion grating (Left), the grating parameters (center), the picture of grooves by SEM (Right)

The limit of the surface flatness in the grooved area is the scale stability of processing machine, and originates in the performance of processing machine, such as temperature stability and atmospheric pressure compensation. Since modification by the holding for machining is added in addition to it, a better surface flatness with around 10nmRMS could be expected in the case of a grating with a higher blaze angles(>50 degrees). Figure 4 shows the surface flatness of the CdZnTe immersion grating in Figure 3.

Figure 4.

Measured surface flatness of grooved area that measured by interferometer in the Littrow condition

2.3

Reducibility

In order to verify a manufacture difference (reproducibility), two devices was manufactured completely independent timing of each. Table 2 shows each manufactured timing. Figure 5 shows the picture of grating for this evaluation. Comparison of the difference was performed by absolute diffraction efficiency and it was checked that the difference is 1% or less. Figure 6 shows the efficiency characteristic dependence on wavelength in the order of 180^th. The outstanding reproducibility has been obtained from the peak wavelength and dependability with both polarizations resembling very closely.

Table 2.

The processing date of two CdZnTe immersion grating

Figure 5.

CdZnTe immersion grating before coating (Left), the edge of grooves by SEM (Center) and after coating (Right)

Figure 6.

Measured absolute diffraction efficiencies.at two gratings for both TE and TM waves

3.1

GRISM

GRISM is the dispersion device, which combined a prism and grating in order to be made to go straight for the specific wavelength at desirable order. The change of an image pick-up and spectrum observation is easy, and since the demand of small size (saving the space) and high efficiency are satisfied, it is very useful device as for a dispersing device. Although ZnSe is most popular material for infrared⁴, since the refractive index is 2.4, the combination flexibility is restricted. Then, realization of GRISM by germanium and Indium Phosphide with the refractive index of 4 and 3.2 respectively is expected.

3.2

Germanium

In germanium, an immersion grating which has a big pitch already has sold and used in the field^{5, 6}. The typical device is called “EGIG” (extreme-high-order echelle Ge grating) and has the pitch of 476μm^{7, 8} or 1111μm. However, since GRISM is used at a low order generally, the pitch of groove becomes a similar to an operating wavelength.

In cutting, the difficulties accompanying by the size of a pitch completely differ. With the GRISM for 2μm of a practical use wavelength limit with 1^st order, a grooved pitch is closed with 2μm and the step difference of groove is especially set to 500nm or less. According our evaluations, it is very difficult for this small pitch and small step difference to perform stable processing from the acceptable tolerance of a diamond tool setting becoming extremely tight. Of course, since in machining which processes one groove at each path, the increase in a number of grooves is directly related with increase of machining time, and serves as restrictions of equipment exclusive time. Although this restriction is equivalent to the ability not to enlarge incidence beam size as an optical element, we made the GRISM with the pitch of 2.2µm for the effective beam diameter of 32mm as a proof of the practical device. The grating parameters is shown in Table 3. This GEISM for the center wavelength of 2.2µm has the design of the spectrum resolution (λ/Δλ) with 4500 at the diameter of 32mm. Figure 7 shows the pictures of Ge GRISM and the grooves. Although processing conditions are severe, in suitable processing conditions, good grooves are obtained. Figure 8 shows measured surface flatness of flat and grooved both surface. It has become lambda/100 or less to design wavelength 2.2µm by RMS. It is the level without consideration about the influence of wavefront error.

Table 3.

The grating parameters

Figure 7.

The picture of Ge GRISM (Left), the picture of grooves by SEM (Right)

Figure 8.

Measured surface flatness of entrance surface and grooved surface

Since Ge is high index material, the antireflection coating is indispensable in order to achieve high efficiency. Figure 9 shows the picture of Ge GRISM after AR coting on both side, with the protected cover for the effective area of both surface and the transport package, respectively. Although the detailed evaluation of diffraction efficiency has not finished yet, it is enough to expect over 90% to the theoretical efficiency or over 60% of absolute efficiency by presumption from measured surface roughness and a number of defects.

Figure 9.

Ge GRISM after AR coating (Left), with protected cover (center) and in the package for transportation (Right)

3.3

Indium Phosphide (InP)

In Indium Phosphide, an immersion grating which has a big pitch already fabricated by machining⁹. Fundamental recognitions of machining are completely the same as germanium for machined GRISM. In our initial investigation, the InP of restrictions is severer. Moreover, since the cover wavelength is shorter than germanium, the detailed investigation of process conditions for InP are required.

We made the GRISM for the center wavelength of 2.2µm with the pitch of 16.9µm for the effective beam diameter of 32mm as a proof of the practical device at this time. The grating parameters is shown in Table 4. Figure 10 shows the pictures of InP GRISM after AR coating on both surface and the grooves. In such large pitch, good grooves are obtained. Figure 11 shows measured surface flatness of flat and grooved both surface. Figure 12 shows the picture of the grooved surface after AR coting on both side and the transport package.

Table 4.

The grating parameters

Figure 10.

The picture of Ge GRISM (Left), the picture of grooves by SEM (Right)

Figure 11.

Measured surface flatness of entrance surface and grooved surface

Figure 12.

The grooved surface with AR (Left), the flat surface with AR (center) and in the case for transportation (Right)