Abstract:
An image forming apparatus includes a light source for emitting light constituting an optical signal, an optical waveguide for conducting the light emitted by the light source and an engine portion for receiving the light conducted by the optical waveguide. The engine portion includes a modulator for modulating the received light and a target on which an image corresponding to the optical signal is formed by being exposed by the modulated light.

Description:
This application claims priority to Japanese Patent Application Nos. H11(1999)-86604 filed on Mar. 29, 1999 and H11(1999)-89732 filed on Mar. 30, 1999, the disclosure of which is incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus for forming an image by being exposed by light, and more particularly to, an image forming apparatus in which light emitted by an exposing light source is conducted by an optical waveguide. 
   2. Description of Related Art 
   In Japanese Unexamined Patent Laid-open Publication No. H9(1997)-277588, an image forming apparatus in which an optical signal is transmitted by an optical fiber is proposed. 
   This image forming apparatus is provided with an optical fiber for transmitting light emitted from an exposing light source such as a semiconductor laser. The image forming apparatus is further provided with a laser driver for driving the light source. The light emitted from the light source driven by the laser driver reaches a polygon mirror via the optical fiber. 
   An optical sensor is provided to control the light amount reaching the polygon mirror so as to be stabilized. The optical sensor monitors the light reflected by the polygon mirror to feed back to the laser driver. A light amount stabilizing controller (APC) in the laser driver  4  controls the light source so as to output a constant optical signal based on the feedback signal from the sensor. 
   However, the aforementioned image forming apparatus has the following drawbacks. Firstly, since the monitored result of the light output from the optical fiber is fed back to the driver of the light source, the light amount stabilizing control is complicated, causing an unstable control system. Secondly, since the light emitted by the light source is transmitted as it is to the photosensitive member for exposing it, the light having the most suitable wavelength is not always transmitted from the view points of a transmission efficiency by the optical fiber and the exposure to the photosensitive member. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to solve the aforementioned drawbacks. 
   It is another object of the present invention to form a high quality and/or high resolution image by an image forming apparatus. 
   It is still another object of the present invention to increase stability of an exposing light amount stabilizing control system in an image forming apparatus. 
   It is still yet another object of the present invention to promote transmission efficiency of exposing light by an optical waveguide in an image forming apparatus. 
   It is still yet another object of the present invention to freely convert a wavelength of exposing light to be transmitted by an optical waveguide. 
   It is still yet another object of the present invention to promote transmission efficiency of exposing light by an optical waveguide in an image forming apparatus and to convert a wavelength of light into a different wavelength of light excellent in exposure sensitivity. 
   According to one aspect of the present invention, an image forming apparatus includes a light source for emitting light constituting an optical signal, an optical waveguide for conducting the light emitted by the light source, and an engine portion for receiving the light conducted by the optical waveguide. The engine portion includes a modulator for modulating the received light and a target on which an image corresponding to the optical signal is formed by being exposed by the modulated light. 
   In this image forming apparatus, the modulator in the engine portion modulates the received light and the modulated light is exposed on the target to form an image corresponding to the optical signal, enabling a light amount stabilizing control, which results in a high-quality and/or high-resolution image forming. 
   According to another aspect of the present invention, an image forming apparatus includes the aforementioned modulator for modulating the wavelength of the light at between the light source and the target. 
   With this image forming apparatus, the wavelength of the exposing light to be conducted by the optical waveguide can be converted. 
   According to still another aspect of the present invention, an image forming apparatus emits light suitable for being conducted by an optical waveguide, and modulates the wavelength of the light into a wavelength corresponding to an optical sensitivity of a target. 
   With this image forming apparatus, transmission efficiency for transmitting the exposing light by the optical waveguide can be increased. Furthermore, the light can be converted into a light having a wavelength excellent in exposure sensitivity. 
   According to still another aspect of the present invention, a printer includes a control portion for generating light constituting an optical signal corresponding to image information, an optical fiber for conducting the light generated by the control portion, and an engine portion. The engine portion includes a receiving portion for receiving the light conducted by the optical fiber, an adjusting portion for adjusting the intensity of the received light and an image forming portion for forming an image corresponding to the optical signal by the light adjusted its intensity, as a unit. 
   According to still another aspect of the present invention, a printer includes a control portion for generating light constituting an optical signal corresponding to image information, an optical fiber for conducting the light generated by the control portion, and an engine portion. The engine portion includes a receiver for receiving the light conducted by the optical fiber, a converter for converting the wavelength of the received light and an image forming portion for forming an image corresponding to the optical signal by the light adjusted its wavelength, as a unit. 
   According to still another aspect of the present invention, an image forming apparatus includes an image writing control portion which receives image information and converts the image information into image forming data, a laser driver for generating a controlling signal based on the image forming data, a laser for oscillating a laser beam constituting an optical signal corresponding to the image forming data based on the controlling signal, an optical fiber which conducts the light oscillated by the laser and outputs the light, a polygon mirror which has a polyhedron configuration and reflects the light conducted by the optical fiber, a photosensitive member for forming a latent image by being exposed by the light reflected by the rotating polygon mirror, an optical output sensor for receiving the light reflected by the polygon mirror to monitor the intensity of the light, and an optical output regulator for regulating the intensity of the light output from the optical fiber based on the intensity monitored by the optical output sensor. The optical output regulator includes a light amplifier for regulating a gain by the controlling light and a gain controller for setting the controlling light based on the intensity monitored by the optical output sensor. 
   Other objects and the features will be apparent from the following detailed description of the invention with reference to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention will be more fully described and better understood from the following description, taken with the appended drawings, in which: 
       FIG. 1  is a block diagram showing an image forming apparatus according to an embodiment of the present invention; 
       FIG. 2  is a schematic structural view showing an image writing control portion in the video controller; 
       FIG. 3  is a schematic structural view showing a print engine control portion in a print engine portion; 
       FIG. 4  is a perspective view showing an image forming portion in the image forming apparatus 
       FIG. 5  is a block diagram showing a construction of an optical output regulator; 
       FIG. 6  is a block diagram showing another construction of optical output regulator; 
       FIG. 7  is a schematic view showing an image forming portion in an image forming apparatus according to another embodiment of the present invention; and 
       FIG. 8  is a perspective schematic view showing a wavelength converter. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a block diagram showing a laser beam printer according to one embodiment of the present invention. 
   As shown in  FIG. 1 , the laser beam printer P is provided with a video controller  3  connected to a host computer  1  as a higher-ranking apparatus by way of a host interface  2  and a print engine portion  5  connected to the video controller  3  by way of a video interface  4  for executing an electrophotographing process. 
   The video controller  3  receives image information transmitted from the host computer  1  to convert the received image information into bit map data, i.e., image forming data for actually recording the image information to an image transfer paper. The video controller  3  is provided with an image writing control portion  6 , a semiconductor laser (such as a laser diode)  7  and a laser driver  8  for driving the semiconductor laser  7 . 
   The aforementioned image information to be transmitted from the host computer  1  includes not only a printing data but also codes for performing a format control and/or a print mode setting. 
   As shown in  FIG. 2 , for example, the image writing control portion  6  includes a microcomputer  61 , a clock generator  62  for driving the microcomputer  61  and an image memory  63  for storing the image information. The image writing control portion  6  converts printing data such as characters transmitted as ASCCII-code into raster-data which are dotted on-off information per every one line. The image writing control portion  6  drives the laser driver  8  in accordance with the raster-data every one line in synchronism with the SOS signal (start of scan signal) sent from the print engine portion  5 . The image writing control portion  6  decodes the print mode data or print format data other than printing data and transmits a control signal to the print engine portion  5  by way of the interface  4  to execute the printing or the formatting. As understood from the above explanation, the image writing control portion  6  receives/transmits various data and/or signals between the video controller  3  and the print engine portion  5 . The signals include an emission permit signal transmitted from the print engine portion  5  for allowing the semiconductor laser  7  to emit laser beam. 
   In this embodiment, a laser diode is used as the semiconductor laser  7 , however, it is not limited to the semiconductor laser. Furthermore, the optical source is not limited to a semiconductor laser. 
   The print engine portion  5  executes an electrophotography process based on the information transmitted from the video controller  3  to record an image on an image transfer paper M (see,  FIG. 4 ). The print engine portion  5  is provided with a print engine control portion  9  which gives and receives signals between the image writing control portion  6  and the print engine control portion  9  and an image forming portion  10  which scans a photosensitive member  22  by an optical signal sent from the semiconductor laser  7  to form a latent image on the photosensitive member  22  which will be mentioned later, and to form an image by executing a developing process, an image transferring process and a fixing process. Furthermore, the semiconductor laser  7  and the image forming portion  10  are optically connected by an optical fiber  11 . 
   As shown in  FIG. 3 , for example, the aforementioned print engine portion  9  is provided with a microcomputer  91  and a clock generator  92  for driving the microcomputer  91 . When a print execute command is issued after the completion of the image information analysis by the video controller  3 , the print engine control portion  9  confirms that the print engine portion  5  is in a standby state. Thereafter, the print engine control portion  9  transmits a light emission permit signal for the semiconductor laser  7  to the video controller  3 . Thus, the electrophotography process starts. 
   Concretely, the microcomputer  91  drives a polygon motor  30  and a photosensitive drum motor  23  by way of a polygon motor control circuit  93  and a drum motor control circuit  94 , respectively. When each motor  30 ,  23  reaches a predetermined rotational velocity, a locking signal is input to keep the rotational velocity unchanged. 
   In order to prevent a partial deterioration of the photosensitive member  22  on a photosensitive drum  24 , the emission permit signal for the semiconductor laser  7  will not be transmitted to the image writing control portion  6  unless the locking signal is activated. 
   The microcomputer  91  is connected by a device (not shown) necessary for driving the laser printer P, input/output devices (not shown) for a sensor. The microcomputer  91  receives a signal from an SOS sensor  33  and transmits a sample and hold signal for controlling a gain of an optical amplifier. 
   As shown in  FIG. 4 , the image forming portion  10  includes a photosensitive drum  24 , an electrostatic charger  25 , an optical output regulator  41  as one of modulators, an optical scanning portion  27  as an latent image forming portion and a developing device  28 . The photosensitive drum  24  comprises a rotating drum  21  and a photosensitive layer (photosensitive member)  22  coated on the outer peripheral surface of the drum  21 , and is rotatably driven by a motor  23 , for example, in the direction of the arrow (a) shown in  FIG. 4 . The optical output regulator  41  is provided at an output end portion of the optical fiber  11 . The optical scanning portion  27  optically scans the photosensitive layer  22  in a horizontal direction thereof (i.e., in the axial direction of the photosensitive drum  24 ) by an optical signal L emitted from the output end  41   a  of the optical output regulator  41  by way of a condenser  26  to form a latent image on the photosensitive layer  22 . The developing device  28  changes the electrical latent image formed on the photosensitive layer  22  by being exposed by the optical signal L into a visual image by toner. 
   The optical scanning portion  27  includes a polygon mirror  31  and an f-θ lens  32  positioned in front of the polygon mirror  31 , etc. The polygon mirror  31  is rotatably driven at a constant high rotational velocity, for example, in the direction of the arrow (b) by a motor  30 , and reflects the optical signal output from the optical output regulator  41  to scan the photosensitive layer  22 . A part of the reflected lay from the polygon mirror  31  advances to an SOS mirror  34 , and is reflected by the SOS mirror  34  to be introduced to an SOS sensor  33 . The SOS sensor  33  functions as a horizontal synchronizing signal detecting sensor for the start of horizontal scanning of the photosensitive layer. The light input to the SOS sensor  33  is photoelectrically converted into an electrical signal which in turn is input into the microcomputer  91  of the print engine control portion  9 . Based on this, the aforementioned SOS signal as a scan start signal is output every one line from the print engine control portion  9  to the image writing control portion  6 . 
   The optical output regulator  41  regulates the output level of the optical signal transmitted through the optical fiber  11  without changing the light itself. In this embodiment, a light amplifier is used as the optical output regulator  41 . The concrete structure is shown in  FIG. 5 . 
   In  FIG. 5 , this optical output regulator  41  includes an erbium added optical fiber (hereinafter referred to as “EDF”) amplifier  44  as a main component to constitute an automatic power control system (hereinafter referred to as “APC”) in which an output is fed back in a real time via a gain control portion  50 . The EDF amplifier  44  controls the gain so as to keep the output of the optical signal L constant by adding exciting light as controlling light. 
   The reference numeral  42  denotes a wavelength division multiplex coupler for coupling the optical signal L transmitted by the optical fiber  11  and exciting light LA which will be mentioned later. Connected to the coupler  42  are a first optical isolator  43 , the EDF amplifier  44 , a second optical isolator  45 , a bandpass filter (hereinafter referred to as “BPF”)  46  for passing only the signal light L and a light-branch coupler  47  for branching a part L 1  of the signal light L in this order. 
   The reference numeral  48  denotes a photodiode as a photoelectric converting element for receiving the branched light L 1  from the light-branch coupler  47 . The reference numeral  50  denotes a gain control portion which outputs the controlling light LA for keeping the output of the EDF amplifier  44  constant depending on the value of the output signal from the photodiode  48 . The gain control portion  50  includes a differential amplifier  51  for comparing the output signal value from the photodiode  48  with a reference value to output the differential and an exciting semiconductor laser  52  using the output of the differential amplifier  51  as injection electric current. The exciting semiconductor laser  52  outputs the exciting light LA as a controlling light corresponding to the comparison output of the differential amplifier  51  to the EDF amplifier  44  by way of the wavelength division multiplex coupler  42 . 
   In other words, the optical signal L transmitted from the video controller  3  by the optical fiber  11  is input into the wavelength division multiplex coupler  42  to be coupled with the exciting light LA from the exciting semiconductor laser  52  by the wavelength division multiplex coupler  42 , and then input into the EDF amplifier  44  by way of the first optical isolator  43 . The EDF amplifier  44  adjusts its gain against the optical signal L depending on the amount of the exciting light LA. The output of the EDF amplifier  44  is input into the BPF  46  via the second optical isolator  45 . The BPF  46  only passes the optical signal L to delete unnecessary natural light. The first and second optical isolators  43 ,  45  restrains unnecessary reflected light to obtain a stable gain. 
   A part L 1  of the optical signal L passed the BPF  46  is converted into an electric signal by the photodiode  48 . The output value E of the electric signal is input into the differential amplifier  51  to be compared with the reference value. The differential thereof becomes injection electric current of the exciting semiconductor laser  52 . 
   If the output value E is larger than the reference value E 1 , the injection current decreases, resulting in a decreased output value of the exciting light LA from the exciting semiconductor laser  52 , which in turn decreases a gain against the optical signal L. On the other hand, if the output value E is smaller than the reference value E 1 , the injection current increases, resulting in an increased output value of the exciting light LA from the exciting semiconductor laser  50 , which in turn increases a gain against the optical signal L. As a result, the gain against the optical signal L in the EDF amplifier  44  is automatically adjusted to be constant. 
   Therefore, even if the characteristic of the semiconductor laser  7  or the optical fiber  11  is uneven when manufactured, or  104  even if the output of the optical signal decreases due to the deterioration of the semiconductor laser  7  or the optical fiber  11  as time passes, the optical output regulator  41  enables a constant output of the optical signal L appropriate for exposing the photosensitive layer  22  at the print engine portion  5 . 
   Next, the operation of the aforementioned laser beam printer P will be explained. 
   When the video controller  3  receives image information from the host computer  1 , the image writing control portion  6  develops the image information into raster data every one line and sends a print execution command to the print engine control portion  5 . 
   The print engine control portion  9  of the print engine portion  5  confirms that the print engine portion  5  is in a standby state based on the print execution command transmitted from the video controller  3 , and transmits an emission permit  25  signal for the semiconductor laser  7  to the video controller  3  to start the execution of the electrophotography process. 
   The video controller  3  starts to operate the laser driver  8  in the video controller  3  based on the raster data when received the emission permit signal for the semiconductor laser  7 . Then, the laser driver  8  operates the semiconductor laser  7 . 
   The semiconductor laser  7  is turned on/off depending on the raster data to output an optical signal L as a laser beam. The optical signal L from the semiconductor laser  7  is transmitted to the image forming portion  10  by way of the optical fiber  11 . As mentioned above, since the semiconductor laser  7  and the laser driver  8  are disposed in the video controller  3 , and the optical signal L emitted from the semiconductor laser  7  is transmitted to the image forming portion  10  via the optical fiber  11 , the transfer loss can be decreased as compared to the case where the semiconductor laser  7  and the laser driver  8  are disposed in the print engine portion  5  and the video signal is transmitted to the laser driver  8  by an electric cable. Furthermore, a high frequency optical signal can be effectively transferred, and emitting noises or receiving external noises can be restrained regardless of the length between the video controller  3  and the image forming portion  10 . 
   The optical signal L transmitted by the optical fiber  11  is adjusted its output level by the optical output regulator  41  at the print engine portion  5 . Thereafter, the adjusted optical signal L is output from the output end  41   a  of the optical output regulator  41 . A part of the optical signal L is branched by the light-branch couple  47  into a branched light L 1  to be photoelectrically converted into an electric signal E. The converted electric signal E is input into a gain control portion  50 . 
   In the gain control portion  50 , the electric signal E is compared with the reference value E 1 . In accordance with the differential, the exciting light LA to be output from the exciting semiconductor laser  52  increases or decreases. 
   The exciting light LA and the optical signal L transmitted by the optical fiber  11  are coupled by the wavelength division multiplex coupler  42 , and the optical signal L is amplified at a certain gain by the EDF amplifier  44 . Only the optical signal of the output of the EDF amplifier  44  is allowed to pass through the BPF  46  to be output from the output end  41   b  of the optical fiber  11 . 
   As explained above, since the output signal level of the EDF amplifier  44  is compared with the reference value and a feedback control for controlling the gain of the EDF amplifier  44  is performed depending on the result of the comparison, the output level of the optical signal L output from the optical output regulator  41  is automatically adjusted to have a constant value corresponding to the reference value E 1 . 
   The optical signal L output from the optical output regulator  41  passes through the lens (condenser)  26 , and is then reflected by the polygon mirror  31  toward the photosensitive drum  24 . Since the polygon mirror  31  is rotating at a constant velocity, the optical signal L horizontally scans the electrostatically charged photosensitive layer  22  of the photosensitive drum  24 . 
   The aforementioned operation is performed every one line based on the SOS signal from the print engine portion  5 , forming a latent image on the photosensitive layer  22  corresponding to the printing data. 
   After the formation of the latent image on the photosensitive layer  22 , a developing process, a transferring process to an image transfer paper M, a fixing process and the like are performed. As a result, a printing data is recorded on the image transfer paper M. 
     FIG. 6  shows another embodiment of the optical output regulator  41 . The detail explanation will be omitted by allotting the same reference numeral to the same or a corresponding portion in  FIG. 5 . 
   In the embodiment shown in  FIG. 6 , a signal from the SOS sensor  33  is input instead of inputting the signal which is a branched part L 1  of the optical signal L and photoelectrically converted by the photodiode  48 . Since this SOS sensor  33  also photoelectrically converts a part of the optical signal L as mentioned above, it can be used as a device equivalent to the photodiode  48 . 
   Concretely, the SOS signal SS is input into the differential amplifier  51  via the sample and hold circuit  61 . Every time the optical signal L is input into the SOS sensor  33 , a sample and hold signal for holding the SOS signal to be input into the differential amplifier  51  at the sample and hold circuit  61  is output from the microcomputer  91  of the print engine control portion  9 . Based on the sample and hold signal, the held signal E is comparted with the reference voltage E 1 , the injection current to the exciting semiconductor laser  52  becomes constant within one line, and a gain control against the optical amplifier  44  is performed. This is an APC type in which a feedback is performed every one line. 
   In the optical output regulator shown in  FIG. 6 , since the existing SOS sensor  33  can be used as a photoelectric converting element, it is not required to branch a part L 1  of the optical signal L or to newly provide photoelectric converting portion for converting the part L 1  of the optical signal L, resulting in a simple structure. 
   Next, an image forming apparatus according to another embodiment of the present invention will be explained. 
     FIG. 7  shows an image forming portion  10  of an image forming apparatus according to the embodiment. The image forming apparatus according to the embodiment has the same elements as in the image forming apparatus P shown in  FIGS. 1–6 , except that the optical output regulator  41  is not provided to the optical fiber  11 ; the light output from the end portion  11   a  of the optical fiber  11  and passed through the lens  26  is input to the polygon mirror  31  via an optical wavelength converter  29  as one of modulators. Therefore, the same portions as in the image forming apparatus shown in  FIGS. 1–6  are allotted by the same reference numerals as in  FIGS. 1–6 , and the explanation will be omitted. 
   In the image forming apparatus shown in  FIG. 7 , the light output from the output end  11   a  of the optical fiber  11  passes the lens  26  and the optical wavelength converter  29  disposed in front of the lens on the optical path in order. 
   The optical wavelength converter  29  converts the wavelength of the optical signal transmitted by the optical fiber  11  into a different wavelength appropriate for exposing the photosensitive layer  22 . In other words, the wavelength excellent for transmitting an optical signal by an optical fiber  11 , or small in transfer loss is different from the wavelength appropriate for exposing the photosensitive layer  22 . Therefore, the wavelength of the optical signal transmitted from the optical fiber  11  is converted into a wavelength coincide with the sensitivity of the photosensitive layer  22  by the optical wavelength converter  29 . 
   As the aforementioned optical wavelength converter  29 , a publicly known structure utilizing, for example, an optical second-harmonic generator may be employed. In this embodiment, an optical waveguide element shown in  FIG. 8  is used. The element has a substrate  51  having a main surface and made of MgO with LiNbO 3  added thereto. Formed on the main surface are an optical waveguide  52  extending along the optical path of the optical signal L, a plurality of reversed polarization regions  53  crossing the optical waveguide  52  and a film  54  having high refractive index coated on the main surface. The optical signal emitted from the output end  11   a  of the optical fiber  11  and introduced into the optical waveguide  52  is partially converted into an optical second-harmonic component due to the nonlinear characteristic of the substrate  51  of the optical waveguide. Furthermore, the reversed polarization region  53  plays a role in adjusting the phase of the optical second-harmonic component. Therefore, the generated second-harmonic components are added to be output as a strong optical signal. As is apparent from the above, new coherent light is generated as a function of the nonlinear polarization by the optical waveguide element, and the wavelength conversion is performed. 
   In this embodiment, the wavelength λ of the optical signal generated by the semiconductor laser  7  and transmitted by the optical fiber  11  is set to 1.55 μm which is small in transfer loss against the optical fiber  11 . The wavelength is converted into a wavelength λ2=775 nm, which is a second-harmonic component and is excellent in exposure sensitivity to the photosensitive layer  22 , by the optical wavelength converter  29 . 
   Since the wavelength of the light generated by the semiconductor laser  7  is set to 1.55 μm which is small in transfer loss against the optical fiber  11 , the loss of the optical signal which is being transmitted by the optical fiber  11  decreases, resulting in a high efficient transmitting of the optical signal. 
   After the optical signal L transmitted by the optical fiber  11  is output from the output end  11   a  of the optical fiber  11 , the optical signal L is input into the optical wavelength converter  29  through the lens  26 . The wavelength small in transfer loss of the optical fiber  11  is converted into a wavelength of 775 nm excellent for exposing the photosensitive layer  22  by the optical wavelength converter  29 . Thereafter, the optical signal is reflected by the polygon mirror  31  toward the photosensitive drum  24 . Since the polygon mirror rotates at a constant velocity, the optical signal L horizontally scans the electrostatically charged photosensitive layer  22  of the photosensitive drum  24 . 
   This operation is repeated every one line based on the SOS signal from the print engine portion  5 , forming a latent image corresponding to the printing data on the photosensitive layer  22 . 
   Since the exposure of the photosensitive layer  22  is performed by the optical signal having a wavelength appropriate to the sensitivity of the photosensitive layer  22 , high resolution latent image or high resolution visual image can be recorded. 
   After the formation of the latent image on the photosensitive layer  22 , printing data are recorded on an image transfer paper M through exposing process, transferring process to an image transfer paper M and fixing process. 
   In the embodiment shown in  FIGS. 7 and 8 , an optical waveguide element is used as the optical waveguide converter  29 . However, the optical waveguide converter  29  may be constituted by any element other than an optical waveguide element. Furthermore, the wavelength of the light before and after the wavelength conversion by the optical wavelength converter  29  is not limited to 1.55 μm and 775 nm. The wavelength may be changed in accordance with the characteristic of the optical fiber  11  or the photosensitive layer  22  to be used. 
   The optical wavelength converter  29  is provided between the output of the optical fiber  11  and the photosensitive layer  22 , and the wavelength of the optical signal output from the optical fiber  11  is converted so as to expose the photosensitive layer  22 . However, the optical wavelength converter  29  may be disposed between the semiconductor laser  7  and the optical fiber  11 . In short, the optical wavelength converter  29  can be disposed in the optical path of the optical signal between the exposure light source  7  and the photosensitive layer  22 . Furthermore, the number of the optical wavelength converter  29  is not limited to one, but may be two or more. 
   In the aforementioned embodiments, the image forming apparatus is explained as a laser printer P. However, the present invention may be applied to any other image forming apparatus such as electrostatic copying machine, etc. 
   The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intent, in the use of such terms and expressions, of excluding any of the equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.