Comparison of Amplitude-based CGH and Phase-based CGH
I. Selection Bottom Line: Determined by the "Material Under Test"
When selecting a CGH type, the primary consideration is the reflectivity of the surface being tested. The core of interferometric testing lies in the "intensity matching" of the reference and test beams; only when the intensities of the two beams are similar will the fringe contrast be optimal.
● Amplitude-type CGH: The First Choice for High-Reflectivity Materials
If your test piece is a metal, single-crystal silicon (Si), modified silicon carbide (SiC), or metallic germanium (Ge), these materials have high reflectivity. Amplitude-type CGHs have relatively low diffraction efficiency (approximately 10%), which perfectly balances the intensity of the high-reflectivity backlight, preventing the interferogram from becoming too bright and causing pixel saturation.
● Phase-type CGH: A Savior for Low-Reflectivity Materials
For materials such as optical glass, quartz, and microcrystalline glass, their surface reflectivity is typically extremely low. If an amplitude-type CGH is used, the backlight signal will be extremely weak and drowned out by background noise. At this point, a phase-type CGH must be used, leveraging its diffraction efficiency of over 40% to "squeeze" every precious beam of signal light, thereby obtaining clear fringes with high contrast.
II. Stray Light Suppression: The "Noise Reduction" Magic of Phase-Type CGHs
In CGH design and testing, the 0th order (transmitted light) and 2nd order (higher-order diffraction) stray lights are extremely difficult to separate. If not handled properly, they will superimpose on the main measurement wavefront, causing artifacts or periodic noise in the surface data.
●Amplitude-type: Energy is distributed across multiple orders, with significant energy in the 0th and 2nd orders, requiring extremely high spatial frequency filtering.
●Phase-type: Through precisely designed etching depth, phase-type CGHs can achieve "energy transfer" at the physical level. It can greatly suppress the energy of the 0th and 2nd orders, concentrating the light energy highly on the desired +1st order measurement wavefront. This inherent frequency selectivity makes the background on the interferometer screen extremely clean, resulting in more reliable measurement data.
Example: Taking an aspherical lens with 4% reflectivity as an example, let's compare the fringe contrast of amplitude-type CGH and phase-type CGH:
Given conditions:
● Interferometer reference surface (TF) transmittance (T_TF): 96%
● Interferometer reference surface (TF) reflectance (R_TF): 4% (as reference light Ir)
● Surface reflectance of the part under test (Rs): 4% (optical glass)
● Contrast formula: V = [2 * sqrt(Ir * It)] / (Ir + It)
1. Amplitude-type CGH: The "Extreme Challenge" of Signal Light rays emitted from the interferometer pass through the reference surface (TF), travel back and forth through the CGH, and finally return to the interferometer:
● Reference light intensity Ir: 4% = 0.04
● Measured light intensity It: It = T_TF * diffraction efficiency_in * Rs * diffraction efficiency_out * T_TF Where the diffraction efficiency is 10%: It = 0.96 * 0.10 * 0.04 * 0.10 * 0.96 = 0.00037
●Stripe contrast V_amp: V = [2 * sqrt(0.04 * 0.00037)] / (0.04 + 0.00037) = 0.189
2. Phase-type CGH: The "golden ratio" of energy
●Reference light intensity Ir: 0.04
●Measured light intensity It: Where the phase-type diffraction efficiency is 40%: It = 0.96 * 0.40 * 0.04 * 0.40 * 0.96 = 0.00589
●Stripe contrast V_phase: V = [2 * sqrt(0.04 * 0.00589)] / (0.04 + 0.00589) = 0.668
●The figure below shows a comparison between the simulated contrast ratios V=0.189 and V=0.668.
Interference fringes of different contrasts
III. Comprehensive Analysis of Processing Routes
Both methods start from the same point, but diverge in the "forming" stage.
1. Amplitude-type CGH: The "Subtractive" Art of Metal Films
● Process: A chromium (Cr) film is sputtered onto a quartz substrate, the pattern is directly written using laser or electron beam, and then wet etching is performed.
● Result: A pattern of alternating "transparent" and "opaque" areas is formed. Its accuracy depends on the positional accuracy of the scribing equipment (Zhixing Optics can achieve 3 sigma <20 nm).
2. Phase-type CGH: The "Engraving" Process of Quartz Substrates
● Process: Based on the amplitude-type method, a key step is added—ion beam etching (IBE) or reactive ion etching (RIE).
● Depth Control: The etching depth must be strictly matched to the detection wavelength. For a wavelength of 633 nm, the etching depth is usually precisely controlled at the nanometer level to ensure that the phase difference achieves optimal light energy distribution and stray light suppression.
CGH Processing Flow Chart
IV. Ningbo Zhixing Optical's Unique Solution: Hybrid Processing Technology
In on-site assembly and adjustment, there is a widely recognized pain point in the industry: while full-phase CGHs have high energy, their alignment area is used for reflection, resulting in low reflectivity and very low fringe contrast, especially when aligning third-order diffraction light, making on-site adjustment extremely difficult.
To address this, Ningbo Zhixing Optical has developed a unique process:
Core Technology: Alignment Area Amplitude + Main Area Phase
● Main Measurement Area (Phase Type): Provides a high-efficiency, low-stray-light measurement wavefront for low-reflectivity materials such as optical glass and microcrystalline materials, ensuring high contrast in the test area.
● Alignment Marking Area (Amplitude Type): Intentionally retains the metallic chromium film structure to enhance the fringe contrast in the alignment area, allowing engineers to instantly capture the light spot during optical path assembly and adjustment, significantly improving work efficiency.
V. Summary: How to choose the right model?
Unity of knowledge and action, boundless optics. Ningbo Zhixing Optics Technology Co., Ltd. focuses on the semiconductor and commercial aerospace fields, providing a complete holographic solution from cross-band measurement to multi-parameter synchronous calibration.
