Laser Head of Laser Engraver

It moves from the center outward in the Table-of-Contents area and finds the specified position on the disc by emitting laser light, and the inductive resistor receives the reflected signal and outputs it as electronic data. The laser head is the heart of the optical drive and the most precise part. It is mainly responsible for reading data, so extra care should be taken when cleaning the inside of the optical drive. The laser head consists of a laser generator (also known as a laser diode), a semi-reflective prism, an objective lens, a lens and a photodiode. When the laser head reads the data on the disc, the laser from the laser generator passes through the semi-reflective prism and converges on the objective lens, which focuses the laser into an extremely small spot and hits the disc. At this point, the reflective material on the disc reflects the irradiated light back through the objective lens and then onto the semi-reflective prism. At this point, because the prism is a semi-reflective structure, it does not allow the light beam to completely penetrate it and return to the laser generator, but is reflected, passes through the lens, and reaches the top of the photodiode. Since the surface of the disc is protruding uneven points to record the data, the light reflected back will be shot in different directions. People will shoot to different directions of the signal is defined as "0" or "1", light-emitting diode to receive those "0", "1 " arrangement of data, and will eventually resolve them into the data we need.

Reading data

Throughout the process of reading data from the laser head, tracing and focusing directly affects the error correction capability of the optical drive as well as its stability. Tracing is the process of keeping the laser head correctly aligned with the track of recorded data at all times. When the laser beam coincides with the track, the tracing error signal is 0. Otherwise, the tracing signal may be positive or negative, and the laser head will adjust the attitude appropriately according to the tracing signal. If the optical drive's tracing performance is very poor, the phenomenon of reading data error will occur when reading the disk, the most typical is the phenomenon of skipping sound when reading the audio track. By focusing, it means that the laser head can precisely hit the beam on the disc and receive the strongest signal. When the laser beam is reflected back from the disc it will hit four photodiodes at the same time. They superimpose the signals and eventually form a focused signal. This signal is only zero when the focus is accurate, otherwise it sends a signal to correct the position of the laser head. Focusing and pathfinding are the two most important properties of the laser head when it works, and what we call a good optical drive for reading discs are products with excellent performance in these two aspects. A few high-grade optical drive products on the market, such as Into the use of stepper motor technology, through the helical screw drive gear, making 1/3 addressing time from the original 85ms to within 75ms, compared to similar 48-speed optical drive products 82ms addressing time, the performance has been significantly improved.

Principle and Structure

Since the introduction of the 12cm diameter digital audio CD in 1982, digital video disc DVD (digitalvideodisk) has been a dream for the new generation of optical discs, although VCD appeared a few years ago, but for optical discs, there is no change in technology, only data compression, and the picture quality is only VHS level, but it is a transitional product, and no market has been formed in It is a transitional product and has not formed a market in foreign countries.

The digital image signal has the advantages of no deterioration in picture quality when being edited and easy to be processed by computer, so the CD-ROM that can record digital images with high picture quality for more than 2 hours has been long-awaited. In recent years, advances in short wavelength semiconductor laser technology, thin optical disc substrate technology, and high numerical aperture NA technology for object lenses have made it possible to record high density optical discs, while digital continuous variable picture compression technology has also made great progress, making it possible to include long, high quality continuous variable pictures on a single disc.

After these technical foundations were laid, ten major companies in the world jointly developed the new generation digital video disc DVD (digitalvideodisk) standard, which is 7.5 times 4.7G recording capacity of the original disc in the same size as the original CD, and uses MPEG2 digital signal compression with high picture quality, enabling it to store 135 minutes of movies.

The DVD player is mainly composed of two key technologies: optical head and MPEG2 decoder, of which MPEG2 decoder is a common standard, and no less than a dozen manufacturers have developed the chip, while the optical head technology is still mainly in the hands of Japanese manufacturers.

The optical disk technology is a minimum laser beam that is focused to the refractive limit and irradiated to the disk surface, and the information on the disk can be read out due to the different reflection of light by the concave and convex of the disk surface where the information is recorded.

Special technology

The following technologies are unique to the optical head: 

1. The signal is extracted from the concave and convex information surface by passing through a 0.6mm transparent plastic layer using the smallest laser beam focused to the bend-back limit.
2. Using a semiconductor laser diode, the laser beam is focused to the smallest beam up to the refractive limit determined by the wavelength using a counter-object lens with a numerical aperture NA of 0.6.
3. In order to automatically compensate for the change in focus position, an error detection function and a servo mechanism for controlling the focus position with an accuracy of plus or minus 1 μm must be built into the optical head.
4. Eccentricity correction between the shape center of the disc and the rotation center of the disc, and error detection function and servo mechanism for controlling the tracing of the laser beam on the track with an accuracy of plus or minus 0.1μm for a track pitch of 0.74μm are built into the optical head.

Here we explain the basic optical system of the optical head that meets the above requirements for the optical device system, the principle and design of the object lens OL (objectlens), the semiconductor laser diode LD (laserdiode) as the light source, the collimation lens CL, and some other optical parts for the optical head.

Basic Principle

The basic optical system of optical head and the difference of optical parts

The optical head is one of the most critical components of the DVD system. The basic principle of the optical head is as follows: 1. object lens, 2. collimating lens, 3. polarizing beam splitting prism, 4. beam splitting prism, 5. reflector, 6. 1/4 wavelength plate, 7. focus error detection optical system, 8. track finding error detection optical system and other optical parts and optical system, 9. focus control servo mechanism (F-ACT), 10. 10. servo-mechanical control parts such as track finding servo (T-ACT), 11. semiconductor laser diode, 12. multi-segment photodiode (PD), and other optoelectronic components.

To focus the laser into the smallest beam determined by the wavelength, the wavefront of the spherical wave emitted from the LD must be transmitted as defect-free as possible to the information recording surface of the disc. In other words, the RMS wavefront convergence of all the optical head imaging system components must be limited to 0.07λ or less from the time the LD emits light to the disc, otherwise the laser beam cannot be focused to the smallest beam determined by the interference limit. The optical components that make up the optical head, the disc surface, including the adjustment error in the setting of the objective lens, must be limited to 0.07λ or less of the maximum permissible wavelength surface difference given by the WarechalCriteron (δω) MC for all of the above combined imaging optics. The optical disc has been specified by the optical disc standard, (δω) DISK = 0.05λ, and the general aberration of the object lens (δω) ADJT = 0.025λ, to make the whole (δω) MC less than 0.07λ, for the other optical parts

The aberration of the other optical parts must be strictly controlled. The maximum allowable wavefront aberration of each optical part of the optical head from LD to CD is expressed by (δω)LD, (δω)CL, (δω)PBS, (δω)QWP, (δω)MR, (δω)OL, and WarechalCriteron gives us the following equation.

(δω)MC≤λ/14

(δω)2MC=(δω)2LD+(δω)2CL+(δω)2PBS+(δω)2QWP+(δω)2MR+(δω)2OL+(δω)2DISK

The following specific DVD values are brought in to try out the calculation. Semiconductor laser diode laser emitting side has a plane glass window, in addition, due to the characteristics of the semiconductor laser itself, insurmountable non-point spacing, worse than the ideal wavefront, ordinary (δω) LD is about 0.013 λ. Prisms, mirrors and other flat optical parts are relatively easy to wavefront collection difference of 0.01 to 0.015 λ manufactured. However, it is difficult to suppress the wavefront aberration within 0.03λ for non-planar optical parts such as collimated lenses and object lenses, which are set at 0.025λ for collimated lenses and 0.035λ for object lenses, respectively. Even if the wavefront aberration of the object lens is suppressed below 0.035, if the wavefront aberration of the collimating lens is greater than 0.025, the diameter of the focused beam will become larger and the frequency of data error from the information surface will become higher. For the above reasons, the wavefront aberration of the collimating lens must be less than 0.025, but it is very difficult to achieve this value with a spherical single lens, and a spherical glass combination lens is generally used.

The laser beam from the counter-object lens of the DVD optical head needs to keep tracking the track with a track pitch of 0.74μm and a shortest crater length of 0.4μm on the information surface of the disc and read out the crater information correctly. The beam diameter at the position where the light intensity is 1/e2 of the beam center intensity is called the beam diameter ω. The laser wavelength λ=650nm and the numerical aperture NA=0.6 for the object lens,

ω=k×(λ/NA)

The coefficient k is 0.96 when the light intensity energy distribution of the incident beam of the object lens is equally distributed, and 1.34 when the light intensity energy distribution is Gaussian. from the above equation, we can see that the beam diameter is proportional to λ/NA, both to improve the disc recording density and reduce the beam diameter, we need to make the laser short wavelength and increase the NA of the object lens.

There is also the focal depth △z of the pair of object lenses is proportional to the square of λ/NA, and the focal depth of DVD becomes 56% narrower compared with CD, and the allowable value of focal error becomes smaller.

The aberration caused by the tilt of the disc will also increase. In order to reduce the aberration caused by the tilt of the disc, the thickness of the disc is reduced to half of that of the CD by 0.6mm.

Imaging Optics

Laser diode

The general LD emits linearly polarized light parallel to the PN bonding surface, but most of the short wavelength LDs emit linearly polarized light perpendicular to the PN bonding surface. DVD requires LDs to have an energy of about 0.3mW on the disc surface, which requires LDs to emit laser energy of 3 to 5mW.

Ejection angle characteristics

LD emitted laser is dispersive light, from the luminous point away from a section of the observed beam cross-sectional intensity distribution, known as the far-field image FFP (farfieldpattern), FFP vertical bonding surface direction is wide, parallel to the direction of the bonding surface is narrow, as shown in the figure below, is longitudinal ellipse. The focal distance of the collimating lens is determined according to the radiation angle of the LD and the beam intensity distribution requirement of the object lens.

Noise characteristics

LD has single-mode luminescence and multi-mode luminescence two kinds of laser hair vibration. The biggest problem of single-mode luminescence is that the light reflected back from the disc enters the laser resonator and forms interference, which becomes noise and affects the SN. The multi-mode LD has strong anti-interference capability and does not require high frequency superposition.

Polarized beam splitting prism

The diverging P-linearly polarized laser emitted from the laser diode passes through the collimating lens, becomes parallel light, and passes through the PBS without reflection and refraction. This parallel circularly polarized light is focused to the information surface of the disc by the object lens, and then reflected back to the disc, and when it passes through the 1/4 wavelength lens, the polarization direction is rotated 45 degrees to become S linearly polarized light, which is reflected to the error detection system and signal system at the PBS of the polarizing beam splitting prism, so that the incident light and the reflected light with signal are separated. The incident light and the reflected light with signal are separated.

Object lens

DVD optical head requires the object lens must be small aberration, excellent characteristics, to focus the beam to the refractive limit, that is, to make up for the various aberrations, so that the size of the point image is completely determined by the refractive limit. Generally, aspheric optical resin lenses are used.

Error detection

Non-point aberration method

Focus error detection method generally uses non-point aberration method, non-point aberration method is based on the change in the position of the reflective surface of the disc, the focus position of the reflected light shifts, through the circular cylindrical surface lens on the shape of the projected light changes, with 4 split PD differential detection.

Focusing error detection signal = (A+C) - (B+D)/(A+B+C+D)

Track finding error detection signal = (A+B) - (C+D)/(A+B+C+D)

PD converts the optical signal into electrical signal, pre-amplifies, analog operation, and then after phase compensation, the signal is fed into the drive amplifier to drive the lens drive coil to complete the focus and track finding control.

Signal system

The reflected light containing information separated from PBS enters the signal system, except for a part of the control system of the servo mechanism, and most of it enters the signal system, which is turned into an electrical signal by PD and pre-amplified to become RF signal.

Cleaning method

Ultrasonic cleaning machine to clean the laser head

Short connect the laser head to protect the solder joints, use a small brush with a blowing balloon to remove the dust on the surface of the laser head, and then use a toothbrush under tap water to remove the grease on the buffer rack. Then put the laser head into the cleaning tank full of warm water, drop in a drop or two of detergent. Let it stand for 10 minutes and then turn on the cleaning machine power and press the cleaning switch. At this time the water in the cleaning tank vibration, the surface of the laser head and the internal bubbles continue to produce, about 5 minutes after the cleaning machine automatically stop. At this time the water in the cleaning tank has been turbid. The cleaned laser head immersed in clean tap water, and then remove the dried inside the sewage (note that the finger pressed objective lens, so as not to damage the objective lens), repeat the implementation of the "immersed - dried" steps four or five times, then the cleaning tank for clean tap water (do not add cleaning solution), and then put the laser head for a second cleaning. To clean the machine automatically shut down, and then the laser head in clean tap water (available pure water or distilled water) for a number of rinsing.

The last process is to remove the internal moisture of the optical head, using a rice cooker (500W) as a drying pot. The method is to wash the rice cooker first, dry the internal moisture, put a heavy bowl in the pot, then turn on the power and press the heating switch. When the heat preservation switch jumps up, lay multiple layers of clean napkins at the bottom of the bowl, then put the dried radical head on the napkins, and then cover the rice cooker cover for about an hour, the water inside and outside the laser head is basically evaporated clean.