Design of the optical system of the optical head

DVD optical head mainly includes objective lens drive system ACT (ACTUATOR) and optical system. Objective lens drive system has two functions, one is to focus the laser emitted from semiconductor laser on the information surface of disc (focusing), the other is to make the light beam on the track and follow the track (tracking). Because focusing is to follow the surface vibration of disc with error less than 1μm, when designing focusing servo drive coil, the acceleration of drive coil must exceed the acceleration of disc surface vibration. Tracking drive coil tracks with error less than 0.1μm. The optical head is used to follow the track. Special attention should be paid to preventing high-frequency mechanical resonance in the design.
The design is generally based on the international standard of the optical disc as the target value, and many factors should be considered, such as the surface vibration of the optical disc, the assembly error of the optical head, the movement error of the optical head, the shaft vibration of the spindle motor, and the placement error of the optical disc. The specific structure mainly includes shaft rotation type and elastic wire support type. The basic characteristics of ACT can be regarded as a spring-assisted advance and retreat structure, and the drive is composed of a coil and an electromagnetic circuit. For specific design, there are computer-aided design software such as PMESH and PMAG.
The following mainly introduces the design of the optical system of the DVD optical head.

Imaging optics
The design of the imaging optics is nothing more than satisfying that the spot after focusing is small enough so that the information on the optical disc can be accurately read. The diameter of the spot is ω=k×(λ/NA), and the coefficient k is related to the incident light intensity distribution to the objective lens. The closer the light intensity distribution is to uniform distribution, the smaller the k value is. Under the condition of the same numerical aperture NA, if you want to get the smallest focused spot,
a. The incident light wavefront aberration is small.
b. The incident light intensity distribution is uniform.
For a, we should strive to make each optical component have the smallest aberration. For b, because the laser emitted from the LD is divergent light, after collimation, the light intensity distribution is Gaussian distribution. If only the laser in the center is used, it can be closer to uniform distribution, but the utilization rate of the LD light energy is low, and the energy required to reach the disc surface may not be obtained. Therefore, it is necessary to compromise these two contradictory requirements and determine the percentage of light intensity distribution Rx (Rim intensity X) used in the horizontal direction of the objective lens and the percentage of light intensity distribution Ry (Rim intensity Y) used in the vertical direction. These two conditions are the basis for designing the optical system.

Laser Diode LD (Laser Diode)
LD is a light-emitting device in an optical head. The characteristics of the light it emits determine the structure and characteristics of the optical head.
a. Basic characteristics
The LD used in a read-only DVD optical head is generally 635nm or 650nm. Its laser resonance threshold current is generally around 40mA, and the working current (that is, when the LD emits laser energy of about 3mW, it is about 50mA. Manufacturers try their best to reduce its working current to make it work at a lower temperature, because temperature changes (increases) will cause the wavelength of the laser it emits to drift. When designing an optical head, it should be designed to get good heat dissipation.
b. Polarization
The laser generated by LD is mostly incomplete linear polarization. When designing PBS, the directionality of linear polarization should be considered.
c. Radiation angle and non-point
interval The divergent laser emitted from the LD semiconductor laser resonant cavity is not emitted from the same point in the horizontal and vertical directions. The distance between the horizontal emission point and the vertical emission point is called the non-point interval, which causes the laser wave surface emitted from the LD to produce non-point aberration.
Since the laser emitted from the LD is divergent light, and the horizontal and vertical divergence angles are different, respectively, θ⊥ and θ∥, the design of the shaping lens and the collimating lens should be based on θ⊥ and θ∥.
d. Oscillation mode and high-frequency superposition and RIN
relative noise intensity RIN is an important indicator for evaluating LD,
RIN=(△P/P)2/△f (unit Hz-1)
where △P is the AC component of the LD emitted light, P is the DC component, and △f is the measured bandwidth.
Multi-mode LDs generally have lower RIN, but have strong anti-reflection light interference capabilities, and the driving current does not need to be superimposed with high frequencies. For single-mode emitting LDs, the reflected light has a strong influence on the optical signal, and high frequencies must be superimposed on the driving current, generally in the range of about 500-700MHZ.
e. Built-in photodiode PD (photo diode)
In order to keep the outgoing light energy constant, a photodiode PD is usually built into the LD for APC (auto power control).

Collimating lens
The collimating lens converts the divergent light emitted by LD into parallel light. Its focal length f is determined by the following three conditions:
a. The longitudinal light intensity distribution Ry of the objective lens.
b. The diameter φ of the objective lens.
c. The vertical radiation angle θ of LD⊥ is determined
by the Gaussian light intensity distribution formula:

The focal length f of the collimating lens can be calculated. Then
a. The focal length f of the collimating lens.
b. The beam diameter required for the objective lens.
c. The formula beam diameter = (objective lens diameter) + (disc eccentricity) + [margin]
2f·NA = beam diameter
The numerical aperture NA of the collimating lens can be calculated.
3.1.3 Shaping prism
Since the laser wavelength used in DVD is short, the light wavefront aberration is strictly required, so the middle part of the LD light emission is used as much as possible, that is, the part with smaller wavefront aberration, and the efficiency of light energy utilization must also be considered. This is different from the general CD optical head. A shaping prism is required, and its function is to transform the elliptical flat light into a perfect circular parallel light (as shown in the figure).

The shaping magnification m and vertex angle of the shaping prism can be calculated based on the following four conditions:
a. The required lateral light intensity distribution Rx
of the objective lens. b. The diameter φ of the objective lens.
c. The longitudinal radiation angle θ∥ of the LD.
d. The focal length f of the collimating lens.

The function of polarizing beam splitter PBS
is to separate the outgoing light from LD and the reflected light from the optical disc, firstly, to prevent the reflected light from returning to the laser resonant cavity of LD so that the outgoing laser does not generate noise, and secondly, to minimize the loss of the reflected laser beam with information. Firstly, the Q surface filters out the components other than the P linear polarization of the incident light A, making it pure P linear polarization. In addition, the Q surface must fully transmit the P linear polarization and fully reflect the S linear polarization.

Objective lens
The design of the objective lens must depend on the thickness of the disc. If the thickness of the disc does not match the design value, spherical aberration will occur, which will deteriorate the focusing characteristics. This is why the objective lens used for a DVD with a disc thickness of 0.6mm cannot read the signal of a CD with a disc thickness of 1.2mm.
The DVDbook stipulates that NA＝0.6, and the radius is generally R＝2mm
fobj=2R/2NA=3.33(mm).
Generally, the read-only type uses a smaller power and can use injection molded aspherical optical resin, while the recording type uses a larger power and is generally composed of multiple groups of optical glass lenses. The aberration of each component of the imaging system must be strictly controlled.

Servo system
Focus servo (Forcs Svero)
There are many ways to obtain the FES focus servo error signal (Forcs Error Single), such as the astigmatism method, the knife edge method, the double knife edge method, etc. Here we only use the astigmatism method, which is relatively simple in the optical system and widely used.
The configuration of the auto focus AF (auto force) optical system is shown in the figure:

Convex lens
The distance between the focus of the convex lens and the focus of the cylindrical lens is called the focal interval D. The detection range on the optical disc is △dsk. The larger the detection range, the lower the sensitivity, but the servo is less likely to derail. On the contrary, the smaller the detection range, the higher the sensitivity, but the servo is easy to derail. D and △dsk are two values ​​that must be determined before designing the focus servo error detection system, and are the basis for design.
β is the horizontal magnification between the focus of the convex lens and the cylindrical lens. There is the following formula:
2β2＝D/△dsk
FAF=βfobj
In this way, the focal length FAF of the convex lens can be calculated.
3.2.1.2. Cylindrical lens

For the incident light A, the light in the m direction intersects the optical axis before the light in the S direction has a distance D. In this way, the j (power) of the cylindrical lens can be calculated.
The refractive index of the convex lens is n1, the thickness is d1, the refractive index of the cylindrical lens is n2, the thickness is d3, the distance between the two lenses is e2, j is the power of the lens, and λ is the wavelength. The others are as shown in the figure above. The curvature radius of the cylindrical lens can be calculated by the following formula:
j=1/F
e=nd
an=an-1＋hn-1j n-1
hn=hn-1－en-1an
3. 2.2. The position and shape design of the photodiode PD (Photo Diodo).
The optical path diagram near PD and the position of PD are as follows (a is the incident light height, F1 is the focal length of the convex lens of the AF system, F2 is the focal length of the cylindrical lens, and x is the position of PD):

a. PD position
The position of PD must be such that the spot lengths in the m and s directions are the same when focusing, i.e. b=b`. The following relationship can be used to determine the position of PD :
a/b=F1/(dx)
a/b`=F2/x . b. PD size The spacing between the 4-split PDs used in ordinary CDs is generally 10mm, but for DVDs, it is generally 5mm to improve accuracy. From the relationship in the above figure, the required PD size can be easily determined. 3.3. Signal extraction system The reflected light with information returned from the optical disc surface is first totally reflected at the PBS, and then divided into servo light and signal light at the beam splitter prism, generally 30% and 70%. The focusing lens in front of the signal PD can be easily determined by the following formula. j=1/F a=an-1＋hn-1jn-1 3.4 Tolerance After the system design is completed, the system tolerance must also be discussed, including the processing error and assembly error of each component. What needs to be discussed is whether the influence of each processing error or assembly error on a certain required index of the optical head is within the permitted range. 3.4.1. Processing error of each component a· Tolerance of shaping prism The evaluation standard of shaping prism tolerance is that the change of Rim intensity in the horizontal direction is within ±1%, and the change parameters are: 1. Change of refractive index of optical glass. 2. Change of LD laser wavelength (affects refractive index). 3. Change of incident angle to shaping prism. 4. Change of top angle of shaping prism. 5. Change of focal distance of collimating lens. 6. Change of radiation angle of LD output light. First, calculate the change of each item from the specification, processing accuracy or adjustment accuracy, and then calculate the change of Rim intensity corresponding to each change. Then, the tolerance can be calculated according to the formula σ=?(e12+ e22+…+en2)/(N-1)?-1/2. en is the amount of Rim intensity change caused by a certain change, and σ is the tolerance of Rim intensity. b. Servo system tolerance The evaluation standard is that the non-point distance is within 5% of the design value, and the detection range of the optical disc is also within 5% of the design value. The change value is: 1. The curvature radius error of the first surface of the convex lens; 2. The curvature radius error of the second surface of the convex lens; 3. The curvature radius error of the first surface of the cylindrical lens; 4. The curvature radius error of the second surface of the cylindrical lens; 5. The distance between the convex lens and the cylindrical lens. 3.4.2. AF system assembly tolerance The evaluation standard is that the movement of the light beam within the detection range does not exceed the deadband value of the PD. Change parameters: 1. Cylinder (convex lens and cylindrical lens in the error detection system are in the same cylinder) eccentricity. 2. Cylinder tilt. 3. Convex lens eccentricity. 4. Convex lens tilt. 5. Cylindrical lens eccentricity. 6. Cylindrical lens tilt. Tolerance can be used to quantitatively evaluate the optical head physically. 3.5 Evaluation of Optical Head The evaluation of the optical head generally measures the intensity distribution and size of the light spot physically, and measures FE, TE, Eye pattern and Jitter in terms of electrical characteristics. The above is mainly the design of the optical path of the optical head. There are also certain design requirements for the frame attached to each optical component in the optical path, such as plane accuracy, adjustability, heat dissipation, rigidity, vibration response characteristics, etc. The design focus of the optical head is different according to the needs of use (such as small optical heads for Disco man, special optical heads for vehicles that are resistant to harsh environments, and high-power optical heads for DVD-R and DVD-RAM, etc.). But the basic design principles are generally the same. The assembly and adjustment of the optical head is also very important. The adjustability of certain parts of the optical head must be considered in the chassis design. Generally, an optical slide axis monitored by a CCD camera is used for adjustment. Sometimes an interferometer is also used to measure the wavefront difference. The DVD optical head combines the latest technologies in semiconductors, lasers, optics, control, mechanics and other fields, and is one of the most important components in DVD optical disc devices.