Image recording apparatus and method of the same

Abstract: A signal LD 36 is driven by a signal LD driver 38 in response to a trigger signal, on which image data are superposed, and emits a pulse laser signal beam modulated by the image data. A pre-amplifier 42 and a power amplifier 46 include optical fibers, which are obtained by doping part of quartz optical fibers with an element such as erbium. Laser excitation beams emitted from excitation LDs are made to pass through and excite the optical fibers. The laser signal beam emitted from the signal LD 36 then passes through the excited optical fibers. This liberates the enhanced energy state and amplifies the intensity of the emitted laser signal beam. A clock generator 30 generates the trigger signal of a fixed period, in response to a control signal output from a control circuit 32. A superposition circuit 28 superposes the image data upon the trigger signal and outputs the resulting superposed signal to the signal LD driver 38. The structure of the present invention realizes highly efficient image recording without using any acousto-optic modulator (AOM) or similar element, and enhances the peak intensity when a thermally sensitive material is irradiated with the laser beam.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a technique of irradiating a thermally-sensitive recording medium with a laser signal beam and recording an image through a thermal reaction, such as thermal ablation, fusion, or sublimation. Description of the Related Art In the technical field of image recording, the number of processes has been reduced as the requirement of the time; for example, the development process has been eliminated. According to this trend, the silver halide photosensitive materials that have been used as the image recording media but require the development process are being replaced by thermally sensitive materials that do not require the development process. Most of the thermally sensitive materials, however, have remarkably poor sensitivities (that is, remarkably large minimum energy densities required for the reaction), compared with the conventional silver halide photosensitive materials. A high-power laser is thus required for irradiating the thermally sensitive material with a laser signal beam and recording an image. The known high-power lasers are solid-state lasers using YAG or YVO4, which, however, have some disadvantages, that is, large-scaled, expensive, and low energy efficiency. In order to overcome such disadvantages, as disclosed in INTERNATIONAL APPLICATION No. PCT/GB93/01760, an image recording apparatus using a fiber laser in place of a solid-state laser has been proposed. The fiber laser used in the proposed apparatus emits a continuous wave laser and thus requires an acousto-optic modulator (AOM) or the like, in order to modulate the emitted laser beam by image data for image recording. The AOM generally includes an acoustic medium for transmitting an ultrasonic wave and an ultrasonic oscillator arranged in the acoustic medium for exciting the ultrasonic wave. The index of refraction of the acoustic medium under the condition that an ultrasonic wave is transmitted through the acoustic medium is different from the same under the condition that no ultrasonic wave is transmitted. When a laser beam is made to pass through the acoustic medium and cross over the transmission pathway of the ultrasonic wave, the laser beam is diffracted in case that the ultrasonic wave is transmitted. In case that no ultrasonic wave is transmitted, on the contrary, the laser beam is not diffracted. Only the diffracted laser beam is then led to the image recording medium. The image recording medium is thus intermittently irradiated with the laser beam in response to the image data. The system of modulating the laser beam with the AOM, however, has some drawbacks. The rise time and the fall time of the signal beam, with which the image recording medium is intermittently irradiated, depend upon the traveling velocity of the ultrasonic wave (carrier) in the AOM and can thus not be shortened to be less than a predetermined time period, for example, 10 ns. Namely the image recording speed can not be increased to be higher than a predetermined level, 40 to 50 Mbps. The resulting recorded image has poor sharpness of lines in the primary scanning direction. The optical system with the AOM requires a complicated and fine adjustment, which worsens the productivity and the workability. Since the laser beam emitted from the fiber laser is generally infrared and the energy required as the carrier (ultrasonic wave) energy in the AOM increases in proportion to the second power of the wavelength of the laser beam, the AOM used for the laser beam emitted from the fiber laser accordingly requires greater carrier energy and is relatively expensive. Another problem also arises when the thermally sensitive material is irradiated with the continuous wave laser beam as discussed above. The thermally sensitive material generally increases its sensitivity (in other words, decreases the minimum energy density required for the reaction) with an increase in peak intensity. For efficient image recording, it is accordingly preferable that the thermally sensitive material is irradiated with the laser beam that attains the higher peak intensity. In case that the thermally sensitive material is irradiated with the continuous wave laser beam, however, the peak intensity, which is defined by the beam output and the beam diameter, can not be sufficiently heightened for efficient image recording. SUMMARY OF THE INVENTION The object of the present invention is thus to provide an image recording apparatus and a method of the same, which realize highly efficient image recording without using any AOM or similar element and enhance the peak intensity when a thermally sensitive material is irradiated with a laser beam. At least part of the above and the other related objects is realized by an image recording apparatus for irradiating a thermally-sensitive recording medium with a laser signal beam modulated by image data and thereby recording an image represented by the image data onto the thermally-sensitive recording medium. The image recording apparatus includes: a first laser for emitting the laser signal beam modulated by the image data; laser signal beam amplification means having an optical fiber doped with a predetermined element and a second laser for emitting a laser excitation beam, the laser signal beam amplification means causing the emitted laser signal beam to pass through the optical fiber that has been excited by the laser excitation beam, thereby amplifying intensity of the laser signal beam; and transmission means for transmitting the amplified laser signal beam to irradiate the thermally-sensitive recording medium with the amplified laser signal beam. The present invention is also directed to a method of irradiating a thermally-sensitive recording medium with a laser signal beam modulated by image data and thereby recording an image represented by the image data onto the thermally-sensitive recording medium. The method includes the steps of: (a) driving a laser based on the image data, thereby causing the laser to emit the laser signal beam modulated by the image data; (b) causing the emitted laser signal beam to pass through an optical fiber that is doped with a predetermined element and has been excited by a laser excitation beam, thereby amplifying intensity of the laser signal beam; and (c) irradiating the thermally-sensitive recording medium with the amplified laser signal beam, so as to record the image onto the thermally-sensitive recording medium. The structure of the present invention uses a laser signal beam modulated by the image data, instead of a continuous wave laser beam, and accordingly does not require any AOM for modulating the laser beam. This simplifies the structure of the apparatus and facilitates the assembly and adjustment, thereby improving the productivity and workability and reducing the cost. Compared with the conventional structure that uses the laser beam modulated with the AOM, the structure of the present invention that uses the laser signal beam directly modulated by the image data has higher rise speed and fall speed. This improves the speed of image recording. In accordance with one preferable application, the image recording apparatus of the present invention further includes superposition means for superposing the image data upon a trigger signal of a fixed period, wherein the laser signal beam emission means emits the pulse laser beam in response to the trigger signal on which the image data is superposed. Compared with the conventional structure using a continuous wave laser beam, this structure enhances the peak intensity and thereby realizes highly efficient image recording. In other word, this structure enables the higher-speed image recording with the laser of the same average output or enables the identical-speed image recording with the lower-power laser. These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates structure of an image recording apparatus as one embodiment according to the present invention; Fig. 2 is a block diagram illustrating detailed structures of the pre-amplifier 42 and the power amplifier 46 of Fig. 1; Fig. 3 illustrates structure of another image recording apparatus using an external drum-type output engine as the image recording unit; Fig. 4 illustrates structure of still another image recording apparatus using a flatbed scanning-type output engine as the image recording unit; and Fig. 5 is a flowchart showing an operational flow of the image recording apparatus of Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS One mode of carrying out the present invention is discussed below as a preferred embodiment. Fig. 1 illustrates structure of an image recording apparatus 20 as one embodiment according to the present invention. Referring to Fig. 1, the image recording apparatus 20 of the embodiment primarily includes a laser signal beam generator unit 22, a laser signal beam amplifier unit 24, a drive/control unit 25, and an image recording unit 34. The laser signal beam generator unit 22 includes a signal semiconductor laser (LD; laser diode) 36 and a signal LD driver 38. The laser signal beam amplifier unit 24 includes two isolators 40 and 44, a pre-amplifier 42, a power amplifier 46, and an excitation LD driver 48. In this embodiment, the laser signal beam generator unit 22 and the laser signal beam amplifier unit 24 constitute a fiber laser using an optical fiber as discussed later. Fig. 2 is a block diagram illustrating detailed structures of the pre-amplifier 42 and the power amplifier 46 of Fig. 1. Referring to Fig. 2, the pre-amplifier 42 and the power amplifier 46 respectively include couplers 74 and 80, excitations LDs 76 and 82, and optical fibers 78 and 84. Referring back to Fig. 1, the drive/control unit 25 includes a bitmap data memory 26, a superposition circuit 28, a clock generator 30, and a control circuit 32. The image recording unit 34 constitutes an inner drum-type output engine and includes an optical fiber 49, a collimator lens 50, a condenser lens 52, a spinner mirror (deflection mirror) 54, a spinner motor 56, an encoder 58, a carrier table 60, a carrier rod 62, a secondary-scanning motor 64, another encoder 68, a drum 70, and a thermally sensitive material 72. The following describes the operation of the image recording apparatus 20 of the embodiment based on Figs. 1 and 2. Image data, that is, the object of image recording, are converted to bit map data, for example, by a raster image processor (not shown) and so on, and stored into the bitmap data memory 26. The image data are successively read from the bitmap data memory 26 in response to a control signal output from the control circuit 32 and are input into the signal LD driver 38 of the laser signal beam generator unit 22 via the superposition circuit 28. Operations of the superposition circuit 28 and the clock generator 30 will be discussed in detail later. The signal LD driver 38 drives the signal LD 36 based on the input image data, and the actuated signal LD 36 then emits a laser signal beam modulated by the image data. By way of example, the signal LD 36 emits the laser beam in response to the high level of image data (on state) and stops the laser beam in response to the low level of image data (off state). The laser signal beam is a trigger of a laser, and an LD having the rated output of approximately 10 mW may be used for the signal LD 36. The laser signal beam emitted from the signal LD 36 passes through the isolator 40, the pre-amplifier 42, the isolator 44, and the power amplifier 46 in the laser signal beam amplifier unit 24 and is eventually led into the optical fiber 49. As shown in Fig. 2, both the pre-amplifier 42 and the power amplifier 46 are constructed as fiber amplifiers to amplify the intensity of the incident laser signal beam. These fiber amplifiers have the optical fibers 78 and 84, which are obtained by doping part of quartz optical fibers with an element such as erbium (Er) or ytterbium (Yb). Laser excitation beams emitted from the high-power excitation LDs 76 and 82 pass through the optical fibers 78 and 84 and excite the atoms in the optical fibers 78 and 84 to enhance the energy state thereof, thereby causing a large population inversion, wherein the number of atoms in the excited state are greater than the number of atoms in the ground state. Under such a condition, the laser signal beam emitted from the signal LD 36 passes through the optical fibers 78 and 84. The laser signal beam accordingly functions as a trigger to liberate the excited energy state as a light beam, so that the laser signal beam is amplified. The excitation LDs 76 and 82 are driven by the excitation LD driver 48 and continuously emit laser excitation beams. The wavelength of the laser signal beam amplified by these fiber amplifiers is 1535 to 1580 nm in case that the optical fibers 78 and 84 are doped with Er, and 1060 to 1130 nm in case that the optical fibers 78 and 84 are doped with Yb. In this embodiment, the maximum output power of the fiber amplifiers are 5 W, whereas the rated output of the excitation LDs are approximately 1W. Each fiber amplifier may thus include a plurality of excitation LDs according to the requirements. The laser excitation beams enter the optical fibers 78 and 84 via the couplers 74 and 80. In case that LDs having the rated output of 1 W are used for the excitation LDs 76 and 82, the emitted laser excitation beams are in a lateral mode. Although such laser excitation beams should enter optical fibers in a multi-mode, the optical fibers 78 and 84 actually used in this embodiment are in a single mode, so that the couplers 74 and 80 have specific structures. The fiber laser consisting of the laser signal beam generator unit 22 and the laser signal beam amplifier unit 24 thus constructed may be YLPM-series provided by the IRE-POLUS group. In the image recording unit 34, the collimator lens 50 converts the laser signal beam led through the optical fiber 49 to a parallel beam of a desired beam diameter, which goes through the center axis of the cylindrical drum 70 and enters the condenser lens 52. The condenser lens 52 collects the incident parallel beam and makes the collected beam reflected by the spinner mirror 54, so that a laser spot is formed on the thermally sensitive material 72 fixed to the inner circumferential surface of the drum 70. The thermally sensitive material 72 is thus irradiated with the laser signal beam modulated by the image data. A thermal reaction, such as thermal ablation, fusion, or sublimation, then proceeds on the thermally sensitive material 72, so as to record an image represented by the image data onto the thermally sensitive material 72. The spinner mirror 54 is driven by the spinner motor 56 to rotate around the center axis of the drum 70, so that the laser spot is scanned in a primary scanning direction on the thermally sensitive material 72. The carrier table 60 with the condenser lens 52 and the spinner mirror 54 mounted thereon is driven by the secondary-scanning motor 64 and moves along the carrier rod 62 arranged in parallel to the center axis of the drum 70, so that the laser spot is scanned in a secondary scanning direction on the thermally sensitive material 72. The carrier table 60 moves by a desired distance at every rotation of the spinner mirror 54, and the laser spot can thus be scanned two-dimensionally on the thermally sensitive material 72. The encoder 58 attached to the spinner motor 56 detects the timing of primary scanning of the laser spot, whereas the encoder 68 attached to the secondary-scanning motor 64 detects the timing of secondary scanning of the laser spot. The control circuit 32 receives the detection signals from the encoders 58 and 68 and generate a variety of control signals based on these detection signals. One of the control signals is input into the bitmap data memory 26 as discussed previously and controls the timing of reading the image data from the bitmap data memory 26. The flowchart of Fig. 5 shows the operational flow of the embodiment discussed above. In the flowchart of Fig. 5, step S10 corresponds to the operation in the laser signal beam generator unit 22, step S12 to the operation in the laser signal beam amplifier unit 24, and step S14 to the operation in the image recording unit 34. The image data read from the bitmap data memory 26 may be input directly into the signal LD driver 38. In this structure, the laser signal beam is modulated only by the image data. This embodiment is, however, preferably provided with the clock generator 30 and the superposition circuit 28, which superpose the image data upon a trigger signal and enable the laser signal beam to be modulated by the resulting superposed signal. The clock generator 30 generates a trigger signal of a fixed period (for example, a sine-wave signal) in response to the control signal from the control circuit 32, and inputs the trigger signal into the superposition circuit 28. The superposition circuit 28 is driven, for example, to output the trigger signal in response to the high-level image data and not to output the trigger signal in response to the low-level image data. The superposition circuit 28 accordingly superposes the image data upon the trigger signal and outputs the resulting superposed signal to the signal LD driver 38. The signal LD 36 is driven by the trigger signal, on which the image data are superposed, so as to produce a pulse wave and emit the pulse laser signal beam modulated by the image data. By way of example, the signal LD 36 repeats the on/off of the laser beam (that is, produces the pulse wave) at the same period as that of the trigger signal in response to the high-level image data, and stops the laser beam in response to the low-level image data. The pulse laser signal beam then enters the fiber amplifiers discussed above. The excitation LDs 76 and 82 in the fiber amplifiers continuously emit the laser excitation beams, which successively excite the atoms in the optical fibers 78 and 84 even during the off period of the pulse laser signal beam and enhance the energy state thereof. During the subsequent on period, the enhanced energy state is liberated at once, which enables the resulting laser signal beam to have extremely high energy. For example, in case that the fiber laser has the average power of 3 W, upon condition that the bit rate (the rate of modulation) is 100 MHz (time period: 10 ns, which corresponds to the period of the trigger signal) and the duration of the pulse wave (the time period when the laser beam is actually emitted) is 2 ns, the intensity of the pulse laser signal beam (the pulse intensity) is equal to 3/(2/10) = 15 W. In this manner, the fiber amplifiers amplify the intensity of the pulse laser signal beam, which is emitted from the signal LD 36. The resulting laser signal beam thus has extremely high energy and attains the extremely high peak intensity on the irradiated thermally sensitive material 72. In this embodiment, the resolution of the image to be recorded is varied by changing the amplification factors in the fiber amplifiers. In accordance with a concrete procedure, the control circuit 32 controls the excitation LD driver 48 to change the intensities of the laser excitation beams emitted from the excitation LDs 76 and 82 in the fiber amplifiers, thereby varying the resolution of image recording. The thermally sensitive material 72 used in this embodiment may be any material that has absorption characteristics at the specific wavelength of the laser signal beam. Especially suitable is an ablation material, such as an LAT (laser ablation transfer) material manufactured by Polaroid Corp., Massachusetts, the USA. As discussed above, the signal LD 36 in the laser signal beam generator unit 22 emits the laser signal beam modulated by the image data, and the structure of the embodiment accordingly does not require any AOM (acousto-optic modulator) that is conventionally used for modulating the laser beam. This simplifies the structure and facilitates the assembly and adjustment, thereby improving the productivity and workability and reducing the cost. Compared with the conventional structure that uses the laser beam modulated with the AOM, the structure of the embodiment that uses the laser signal beam directly modulated by the image data has higher rise speed and fall speed. This improves the speed of image recording and enhances the sharpness of lines in the primary scanning direction in the resulting recorded image. In the embodiment, since the LD of relatively low output (approximately 10 mW) is used for the signal LD 36, direct modulation of the laser signal beam by the image data realizes high-speed modulation (of not lower than 100 Mbps). The structure of the embodiment includes the clock generator 30 and the superposition circuit 28, which enable the pulse laser signal beam to be emitted from the signal LD 36. The fiber amplifiers then amplify the intensity of the emitted pulse laser signal beam, and the thermally sensitive material 72 is irradiated with the amplified laser signal beam. Compared with the conventional structure using the continuous wave laser beam, the structure of the embodiment heightens the peak intensity and shortens the exposure time. The thermally sensitive material 72 generally has the better sensitivity for the higher peak intensity and the shorter exposure time. This is because the shorter exposure time decreases the thermal energy released from the position irradiated with the laser signal beam to the periphery (that is, the loss of thermal energy) among all the thermal energy diffusion to the thermally sensitive material 72. The structure of the embodiment that uses the pulse laser signal beam improves the sensitivity of the thermally sensitive material 72 and thereby realizes highly-efficient image recording. By way of example, this structure enables the higher-speed image recording with the laser of the same average power or enables the identical-speed image recording with the lower-power laser. The present invention is not restricted to the above embodiment, but there may be many modifications, changes, and alterations. In the above embodiment, the inner drum-type output engine is adopted as the image recording unit 34. The principle of the present invention is, however, not limited to this structure, but an outer drum-type output engine may be used as an image recording unit 34' as shown in Fig. 3. Fig. 3 illustrates structure of another image recording apparatus 20' using an outer drum-type output engine as the image recording unit 34'. In the image recording apparatus 20' of Fig. 3, the image recording unit 34' includes an optical fiber 81, a carrier table 83, a condenser lens 85, a carrier rod 86, a secondary-scanning motor 88, an encoder 90, a drum 92, a primary-scanning motor 94, and another encoder 96. The image recording apparatus 20' shown in Fig. 3 has the same constituents as those of the image recording apparatus 20 shown in Fig. 1, except the image recording unit 34', and the structures and operations of these constituents are not specifically described here. In the image recording unit 34' of Fig. 3, the condenser lens 85 collects the laser signal beam led through the optical fiber 81 and enables a laser spot to be formed on a thermally sensitive material 98 fixed to the outer circumferential surface of the drum 92. The primary-scanning motor 94 rotates the drum 92 around the center axis thereof, so that the laser spot is scanned in the primary scanning direction on the thermally sensitive material 98. The carrier table 83 with the condenser lens 85 mounted thereon is driven by the secondary-scanning motor 88 and moves along the carrier rod 86 arranged in parallel to the center axis of the drum 92, so that the laser spot is scanned in the secondary scanning direction on the thermally sensitive material 98. The encoder 96 attached to the primary-scanning motor 94 detects the timing of primary scanning of the laser spot, whereas the encoder 90 attached to the secondary-scanning motor 88 detects the timing of secondary scanning of the laser spot. The control circuit 32 receives the detection signals from the encoders 96 and 90 and generate a variety of control signals based on these detection signals. The principle of the present invention is not restricted to the drum-type output engines shown in Figs. 1 and 3, but a flatbed scanning-type output engine may be used as an image recording unit 34" as shown in Fig. 4. Fig. 4 illustrates structure of still another image recording apparatus 20" using a flatbed scanning-type output engine as the image recording unit 34". In the image recording apparatus 20" of Fig. 4, the image recording unit 34" includes an optical fiber 100, a base table 102, an auxiliary deflector 104, a collimator lens 106, a polygon mirror 108, an f lens 110, a roller 112, a thermally sensitive material 114, mirrors 118 and 120, a start sensor 122, and an end sensor 124. The image recording apparatus 20" shown in Fig. 4 has the same constituents as those of the image recording apparatus 20 shown in Fig. 1, except the image recording unit 34", and the structures and operations of these constituents are not specifically described here. In the image recording unit 34" of Fig. 4, the laser signal beam led through the optical fiber 100 enters the polygon mirror 108 via the auxiliary deflector 104 and the collimator lens 106. The polygon mirror 108 rotates at a fixed rate in the direction of the arrow and successively reflects the incident laser signal beam by eight mirrors attached to the eight outer circumferential faces thereof. This enables sector scanning of the incident laser signal beam. The laser signal beam reflected by the polygon mirror 108 is collected by the f lens 110 and enables a laser spot to be formed on the thermally sensitive material 114 wound on the roller 112. Out of the effective range of the image data, the laser beam continuously emitted from the signal LD 36 is received by the start sensor 122 and the end sensor 124 via the mirrors 118 and 120. The start sensor 122 accordingly generates a start detection signal representing the timing of a start of one primary scanning cycle, and the end sensor 124 generates an end detection signal representing the timing of an end of one primary scanning cycle. The control circuit 32 receives these detection signals and calculates the exposure time of one primary scanning cycle from the start time and the end time of one primary scanning cycle, so as to control the timing of laser irradiation. It should be clearly understood that the above embodiment is only illustrative and not restrictive in any sense. The scope of the present invention is limited only by the terms of the appended claims.

Claims: An image recording apparatus for irradiating a thermally-sensitive recording medium with a laser signal beam modulated by image data and thereby recording an image represented by said image data onto said thermally-sensitive recording medium, said image recording apparatus comprising: a first laser for emitting said laser signal beam modulated by said image data; laser signal beam amplification means having an optical fiber doped with a predetermined element and a second laser for emitting a laser excitation beam, said laser signal beam amplification means causing said emitted laser signal beam to pass through said optical fiber that has been excited by said laser excitation beam, thereby amplifying intensity of said laser signal beam; and transmission means for transmitting said amplified laser signal beam to irradiate said thermally-sensitive recording medium with said amplified laser signal beam. An image recording apparatus in accordance with claim 1, said image recording apparatus further comprising superposition means for superposing said image data upon a trigger signal of a fixed period, wherein said first laser emits said pulse laser signal beam in response to the trigger signal on which the image data is superposed. An image recording apparatus in accordance with claim 1, wherein said first laser is driven based on said image data. A method of irradiating a thermally-sensitive recording medium with a laser signal beam modulated by image data and thereby recording an image represented by said image data onto said thermally-sensitive recording medium, said method comprising the steps of: (a) driving a laser based on said image data, thereby causing said laser to emit said laser signal beam modulated by said image data; (b) causing said emitted laser signal beam to pass through an optical fiber that is doped with a predetermined element and has been excited by a laser excitation beam, thereby amplifying intensity of said laser signal beam; and (c) irradiating said thermally-sensitive recording medium with said amplified laser signal beam, so as to record said image onto said thermally-sensitive recording medium.

Inventors: TAMAKI EIICHI, TAMAKI, EIICHI, TAMAKI, EIICHI, C/O DAINIPPON SCREEN MFG. CO. LTD.
Applicants: DAINIPPON SCREEN MFG, DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1997
Main classification: B41J2
IPC codes: B41J2, B41M5, G02B26