JPH053564B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <<Industrial Application Field>> The present invention relates to an optical scanning device that uses a hologram to deflect the direction of a light beam at high speed. More specifically, the present invention provides an optical system that is effective for use in devices that record and display signals or images using optical signals, and that stabilizes the wavelength of a semiconductor laser light source used by signal modulation. The present invention relates to a scanning device.

<<Prior Art>> FIG. 2 is a block diagram of main parts of a conventional optical scanning device using a hologram. This device uses object light 1 as diverging light from a point light source and reference light 2 as parallel light (both shown by broken lines) to make parallel beams incident on holograms 31, 32, 33, etc., which have been exposed and produced. , so that a reproduced image is obtained by reproduction light 5 on an imaging plane 4 which is a scanning plane disposed behind the hologram disk 3.

As an application of hologram scanner, POS
(Point of Sales) barcode readers, laser printers, etc. can be considered. The latter case, such as a laser printer, uses a semiconductor laser as a light source by modulating it.

Conventional laser printers use polygons and
Mirrors were often used, but it is desired to replace them with hologram scanners, which are superior in terms of mass production and cost.

<<Problems to be Solved by the Invention>> However, when a hologram scanner is used in a system that modulates a semiconductor laser, the following problems occur.

FIGS. 3 and 4 show wavelength-temperature characteristics of typical semiconductor lasers, and show that the wavelength of the semiconductor laser changes with temperature. Such a semiconductor laser is modulated by the conventional method shown in Figure 5 (for example, when the recording signal is a logic 1, a constant current is modulated).
Flow Iop to emit light, and when the logic is 0, the current is 0
Then, the amount of heat generated by the semiconductor laser changes depending on the modulation signal, so the temperature of the semiconductor laser changes, and as a result, the oscillation wavelength of the semiconductor laser changes. When using a mirror for optical scanning,
This wavelength change does not affect the scanning position, but in the case of a hologram scanner that utilizes a diffraction phenomenon, the scanning position changes due to the wavelength change.

The amount of change in scanning position is calculated as follows. Assume that the wavelength of the semiconductor laser is λ _r , the angle of incidence of the laser beam on the hologram/scanner is θ _r , the interference fringes on the hologram are perpendicular to the plane of incidence, the spatial frequency is f , and the diffraction angle of the laser beam is Let θ _d be the following relationship.

sinθ _d = fλ _r − sinθ _r ……(1) Therefore, the change in the diffraction angle △θ _d when the wavelength λ _r changes by △λ _r is △θ _d = f△λ _r /cosθ _d …… (2) becomes. Here, f = 1800 (lines/mm), θ _d = 45
(deg) (value of linear grid hologram scanner),
If △λ _r =0.5×10 ^-6 (mm), the rotation angle changes by △θ _d =1.27×10 ^-3 (rad). Considering the case where it is applied to a laser printer, it is
Assuming that the photosensitive drum is located at mm, the change Δx in the position of the laser beam on the drum in this case is Δx≒300×Δθ _d =0.38−mm−.

In a conventionally modulated semiconductor laser, the wavelength change of about 0.5-nm used in the above calculation actually occurs, and the resulting positional change of the laser beam of about 0.38-mm is the difference between a laser beam and a printer. This becomes a serious problem when considering applications. Furthermore, since this wavelength change occurs discontinuously as a mode hop, a printed straight line is broken into steps, as shown in FIG. 6, for example.

<Means for Solving the Problems> A device according to a first aspect of the present invention is an optical scanning device using a transmission type or reflection type hologram disk, which includes a semiconductor laser that irradiates the hologram surface, and a semiconductor laser that irradiates the hologram surface. The present invention is characterized by comprising a modulation circuit that modulates the semiconductor laser at a frequency that is sufficiently faster than the thermal response time of the semiconductor laser when emitting light, and flows a bias current that is less than a threshold current of the semiconductor laser when the semiconductor laser is not emitting light.

A device according to a second aspect of the present invention is characterized in that a temperature control section for keeping the ambient temperature of the semiconductor laser constant is added to the above device.

<<Operation>> According to the device having the above structure according to the first aspect of the invention, it is possible to equalize the average amount of heat generated by the semiconductor laser when emitting light and when not emitting light.

According to the device having the above configuration according to the second aspect of the invention, by keeping the temperature related to the semiconductor laser constant, the wavelength of the semiconductor laser can be stabilized.

<<Example>> The present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an optical scanning device using a hologram according to the present invention. 10
is an input terminal for inputting a binary recording signal _e1 , 11
is a modulation circuit that modulates the signal from this input terminal 10, and 12 is a modulation current output from this modulation circuit 11.
i ₁ is a semiconductor laser driven by 1; 13 is a lens that appropriately converges the output light from this semiconductor laser 12; 3 is a lens that passes through this lens 13, and the reproduction light is irradiated onto the hologram surface, and is A rotating hologram disk; 4 is an imaging surface on which the diffracted light 14 is scanned as the hologram disk 3 rotates; 15 is a mount to which the semiconductor laser is mounted; 16 is a heat insulating material such as polystyrene foam that covers the mount 15; 17 is a temperature detector such as a transistor temperature sensor that detects the temperature of the mount 15; 18 is this temperature detector 17;
The temperature control circuit 19 inputs the detection signal from the temperature control circuit 18 is a heat absorber or heat absorber that performs heat absorption or heat generation based on the output signal from the temperature control circuit 18, and uses a combination of a Peltier element and a heat radiator, for example. The temperature detector 17, the temperature control circuit 18, and the heat absorber 19 constitute a temperature control section 30.

The operation of the apparatus configured as described above will be explained next. FIG. 7 is a time chart for explaining the operation of the modulation circuit 11. When a binary recording signal e ₁ as shown in FIG. 7A is applied to the modulation circuit 11 via the input terminal 10, a modulation current output i ₁ as shown in FIG. added to. The modulated current output i ₁ in FIG. 7B is a current (amplitude Ia) modulated at a frequency faster than the thermal response of the semiconductor laser 12 when emitting light, and a bias current Ib below the threshold current of the semiconductor laser 12 when not emitting light. Flow. If the peak current value during light emission is Ia, the operating voltage of the semiconductor laser is Va (almost unchanged even if the current changes), and the modulation duty ratio is Dr, then the difference in the average heat generation amount during light emission and non-light emission is Wc is Wc=DrIaVa−IbVa (3), and the peak current value Ia and duty ratio Dr
By appropriately selecting , it is possible to make the difference in average calorific value Wc to 0. Considering the optical output, equation (3) becomes Wc=DrIaVa−DrPa−IbVa (4). However, Pa is the peak light output during light emission, which is about several percent of the normal power consumption, and below the threshold current, the light output is so small that it can be ignored.

The light emitted from the semiconductor laser is appropriately focused by a lens and is incident on the hologram disk. The incident light is diffracted by the hologram, and as the hologram disk 3 rotates, the diffraction angle changes, and as a result, the laser beam 14 is scanned.

Mount 15 to which semiconductor laser 12 is attached
The temperature is monitored by a temperature detector 17, and feedback controlled by a temperature control circuit 18 and a heat absorber 19 to keep it constant. mount 15
In order to maintain the temperature of the semiconductor laser 1 at around room temperature,
Since it is only necessary to absorb the heat generated in step 2, the heat absorbing device may have a simple structure that only performs heat absorption.

The temperature control section described above also performs the following functions. That is, in the step-like wavelength-temperature characteristic shown in FIG. 3, the ambient temperature of the semiconductor laser can be controlled so as to avoid the temperature B at which the wavelength becomes unstable and reach the stable temperature A.

According to a device having such a configuration, when the semiconductor laser is signal-modulated, the heat generation of the semiconductor laser becomes constant and the ambient temperature of the semiconductor laser is kept constant and stable, so that the output light from the semiconductor laser is kept constant. Stabilize the wavelength. As a result, the optical scanning device can perform stable scanning without changing the scanning position due to changes in the wavelength of the laser beam.

FIG. 8 is a perspective view of a main part showing the state of the optical scanning section (same as the optical scanning device of Japanese Patent Application No. 58-150755) in the apparatus shown in FIG. 1 during recording (fabrication) and reproduction (scanning). In these figures, numeral 3 denotes a hologram disk (light scanning section), on which a plurality of holograms 3 ₁ , 3 ₂ , 3 ₃ . (arrow a
(direction shown). This hologram disk 3 rotates (moves) in the direction of arrow a, and can perform one scan with one hologram. Note that here, an example using a transmission type hologram disk is shown. 4 is an imaging plane.

First, the object light source 22 used in producing the hologram uses a convergent spherical wave, and its convergence point is at point O. Further, the reference light source 21 has the same wavelength as the object light source 22 (for example, wavelength λ _C =0.63 μm).
A diverging spherical wave is used, and its point light source is at point R. These object light source 22 and reference light source 21 are
Both are installed so that the incident angle of the light beam emitted from the hologram to the hologram is at an angle other than 0° at the scanning center, that is, at an angle, and the light from each of these light sources causes the hologram 3 ₁ , 3 ₂ , 3 ₃ …
Interference fringes are recorded on top.

A reproduction light source 20 (corresponding to the semiconductor laser 12 and lens 13 in FIG. 1) used during reproduction (scanning) emits a diverging spherical wave of a wavelength different from that of the reference light (for example, the wavelength λ _R of the semiconductor laser light = 0.78 μm). is used, and the point light source is at point Q, which is at a different position from the point light source R, which is the reference light source. This reproduction light source 20
The light beam from the hologram is incident on the hologram disk 3 at an angle other than 0° at the scanning center, that is, obliquely, and the reproduced light from the hologram is transmitted to each hologram 3 ₁ , 3 ₂ , 3 ₃ It is diffracted with high diffraction efficiency by interference fringes recorded in advance on... As the hologram disk 3 rotates (moves), this diffracted light scans a small optical spot on the imaging plane 4 in a straight line from S ₁ to S ₂ to S ₃ without aberration.

Here, in the main optical scanning section, the reference light source 21
The position of the reproduction light source 20 and the focal position of the object light source 22 are selected at predetermined positions so as to satisfy the following equations (5) and (6), thereby achieving linear, aberration-free scanning. are doing. At the same time, the angle of incidence of the reproduction light on the hologram is selected to be equal to or close to the Black angle of incidence. That is, high diffraction efficiency is obtained by satisfying the Bragg condition of equation (7).

f _C (〓 _Cp ,〓 _CR , r, θ,, λ _C )≒f _R
(〓 _RO , 〓 _RR , r, θ, , λ _R ) ...(5) φ _C (〓 _Cp , 〓 _CR , r, θ, , λ _C )≒
φ _R (〓 _Rp , 〓 _RR , r, θ,, λ _R ) ……(6) θ _r ≒ sin ^-1 (λ _R /2d′) (θ _pd −θ _ref )/2 ……(7) However , f _C : Spatial frequency of interference fringes recorded on the hologram surface by the object beam and reference beam φ _C : Inclination of the above interference fringes f _R : Virtual light flux and reproduction light converging on a predetermined point on the imaging plane Therefore, the spatial frequency of interference fringes formed on the hologram surface φ _R : Inclination of the above interference fringes〓 _Cp : Vector indicating the light source position of the object beam〓 _CR : Vector indicating the light source position of the reference light〓 _Rp : Light source of the virtual light flux Vector indicating the position〓 _RR : Vector indicating the light source position of the reproduction light (r, θ): Coordinates on the hologram disk surface: Rotation angle of the hologram disk λ _C : Wavelength of the recording light (reference light, object light) λ _R : Wavelength of reproduction light and virtual beam θ _r : Incident angle of reproduction light θ _pb : Incident angle of object beam θ _ref : Incident angle of reference beam d' : Three-dimensional pitch of interference fringes on hologram disk Object beam and reference Hologram 3 ₁ , 3 ₂ by light
The light source positions of the object beam, reference beam, and reproduction beam are set so that the incident angle of the reproduction light satisfies the bragg condition and the linearity and convergence are the best for the interference fringes created at the center point P of... It can be found from equations (5), (6), and (7), but in reality, equations (5) and (6) are difficult to solve analytically. Therefore, a method is used in which equations (5), (6), and (7) are replaced with extreme value problems, and this is solved by a numerical method using a computer.

FIGS. 9 and 10 are characteristic diagrams obtained by examining the linear error of the scanning spot obtained on the imaging plane 4 and the convergence spot diameter, respectively, in the apparatus according to the above embodiment. In this experiment, a He-Ne laser with a wavelength λ _C of 0.63 μm was used as the reference light source, and a semiconductor laser with a wavelength λ _R of 0.78 μm was used as the reproduction light source, and a scanning width of ±150 mm was obtained. Within this range, satisfactory results were obtained with linearity of ±100 μm and maximum spot diameter of φ100 μm.

Note that although a transmission type hologram disk is used in the above embodiment, a reflection type hologram may be used instead. Further, by using a photoresist as a photosensitive material, after exposure and development, the substrate is etched by oblique ion etching to form an eshlet-shaped grating to improve the diffraction efficiency. In this case, it is sufficient to perform exposure under conditions that satisfy only equations (5) and (6). In addition, using Si crystal etc. for the substrate,
Anisotropic etching can also be used to create an eshlet-shaped lattice. Furthermore, the above-mentioned echelon grid may be used as an original and a replica may be made using transparent plastic or the like.

Further, the optical scanning section is not limited to the one used in the above embodiment, but a conventional type such as the apparatus shown in FIG. 2 can also be used.

In the embodiment shown in FIG. 1, the output of the temperature detector 17 is used as the temperature detection signal in the temperature control section, but the modulation circuit 11
If the amount of light at the time of light emission is controlled to be constant (by performing feedback using a light amount sensor or the like), the laser current at the time of light emission can also be used as a temperature detection signal in the temperature control section. This takes advantage of the fact that the current-optical output characteristics of a semiconductor laser change depending on temperature, and the driving current required to obtain a constant optical output increases as the temperature rises. In this case, the temperature detector 17 becomes unnecessary.

<<Effects of the Invention>> As described above, according to the present invention, it is possible to realize an optical scanning device with a simple configuration that does not cause errors in the scanning position due to changes in the wavelength of the semiconductor laser and has stable scanning.

[Brief explanation of the drawing]

FIG. 1 is a block configuration diagram showing an embodiment of the device according to the present invention, FIG. 2 is a configuration diagram of main parts of an optical scanning device using a conventional hologram, and FIGS. 3 and 4 show wavelengths of semiconductor lasers. A characteristic curve diagram showing temperature characteristics, FIG. 5 is a time chart showing a conventional binary modulation method of a semiconductor laser, and FIG. 6 is a diagram showing a shift in scanning position that occurs when using the conventional modulation method.
FIG. 7 is a time chart for explaining the operation of the apparatus shown in FIG. 1, FIG. 8 is a main part configuration diagram for explaining the operation of the optical scanning section in the apparatus shown in FIG. 1, and FIGS. 9 and 10. is a characteristic curve diagram of the device according to the present invention. 3...Hologram disk, 11...Modulation circuit, 12...Semiconductor laser, 30...Temperature control section, Ib...Bias current.

Claims (1)

[Claims] 1. In an optical scanning device using a transmission type or reflection type hologram disk, there is provided a semiconductor laser that irradiates the hologram surface, and a frequency that is sufficiently faster than the thermal response time of the semiconductor laser when emitting light from the semiconductor laser. and a modulation circuit that modulates the semiconductor laser with a bias current equal to or lower than the threshold current of the semiconductor laser when the semiconductor laser is not emitting light, so that the average amount of heat generated by the semiconductor laser when the semiconductor laser is emitting light and when it is not emitting light is made equal. Optical scanning device used. 2. In an optical scanning device using a transmission type or reflection type hologram disk, a semiconductor laser that irradiates the hologram surface, and a semiconductor laser that is modulated at a frequency sufficiently faster than the thermal response time of the semiconductor laser when emitting light and modulated when not emitting light. A modulation circuit that equalizes the average heat generation amount of the semiconductor laser when emitting light and when not emitting light by flowing a bias current that is less than a threshold current of the semiconductor laser; and a temperature control circuit that keeps the temperature related to the semiconductor laser constant. An optical scanning device using a hologram, characterized in that the wavelength of a semiconductor laser is stabilized.