JP4914083B2 - Optical wavelength conversion device, optical wavelength conversion method, and image forming apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light wavelength conversion device for stably controlling high gradation without depending on a modulation signal pattern and to provide a light wavelength conversion method and a control method of the device. <P>SOLUTION: The light wavelength conversion device comprises a DBR laser 101, a light wavelength conversion element 104 which makes a fundamental wave light emitted from the DBR laser 101 incident, and outputs second harmonic light, and a control means 109 controlling the DBR laser 101. The control means 109 injects constant driving current different in two time domains to either an active region 101a or a DBR region 101c, injects binary driving current in the two time domains to the other region when one period of driving current is divided into the two equal time domains, and adjusts a current value of driving current and injection time so that a sum of heating values generated in one period becomes almost constant at every period by driving current injected to the other region. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

The present invention relates to an optical wavelength conversion apparatus or method for converting semiconductor laser light into second harmonics. In particular, the present invention relates to an optical wavelength conversion apparatus or method for emitting laser light that can be used for high-speed modulation driving and is used as a light source for laser display, electrophotographic image formation, optical recording, optical measurement, and the like. The present invention also relates to a method for controlling such an optical wavelength converter, and an image forming apparatus such as a laser display and a laser beam printer using the same.

Semiconductor lasers are used in various fields such as optical communication systems, CD / DVDs, and measuring instruments, taking advantage of their small size, high output, and low cost. However, blue-violet lasers have been put into practical use only in recent years, but semiconductor lasers in the wavelength band of green or lower than the ultraviolet region have not yet been commercialized. Expectations are high for green, which is one of the three primary colors, and for small lasers with short wavelengths and high powers that are applied to laser processing machines and high-density optical memories.

Against this background, various methods using second harmonic generation (SHG) have been proposed as methods for obtaining a short wavelength laser light source. Generally as a light wavelength conversion element (SHG element), periodically poled lithium niobate (PPLN;
Periodically Poled Lithium Niobate) is used. In addition, since the wavelength selection width of SHG elements is usually as narrow as 1 nm or less, DFB (Distributed) has good single-mode characteristics and wavelength stability as the fundamental light source.
Feedback) laser and DBR (Distributed Bragg Reflector) laser are used.

When a modulation current is injected into such a semiconductor laser, there is a problem that the oscillation wavelength varies due to the thermal history depending on the modulation pattern, and as a result, the output of the SHG light also varies. For this reason, an SHG laser light source considering the problem has been disclosed (see Patent Document 1). FIG. 16 is a diagram showing the configuration. As shown in FIG. 16, the SHG laser light source includes a DBR semiconductor laser 810 having a DBR region 813, a phase adjustment region 812, and an active region 811; an SHG element 820; and a drive circuit 830. The drive circuit 830 supplies the amount of current injected into the active region 811 and the phase region 812 so that the sum of the amount of heat transferred from the active region 811 to the DBR region 813 and the amount of heat transferred from the phase region 812 to the DBR region 813 is constant. The amount of injected current is controlled. As a result, the wavelength of the fundamental light from the DBR semiconductor laser 810 is stabilized, and the dependency of the SHG light (second harmonic light) from the SHG element 820 on the modulation pattern is reduced.

In addition, in an SHG laser light source equipped with a DBR semiconductor laser and an optical wavelength conversion element, the current values of the DBR region, active region, and phase region are set to obtain a desired harmonic output and to stabilize the output. There is also a proposal for a control method (see Patent Document 2).
JP 2002-43698 A Japanese Patent No. 3329446

According to the method disclosed in Patent Document 1, the temperature change of the DBR region 813 due to the modulation current pattern is reduced to some extent. However, since the fundamental light is modulated by inputting a modulation current to the active region 811, the light energy input to the SHG element 820 varies. For this reason, the temperature of the SHG element 820 varies, and the phase matching wavelength is not stabilized. As a result, the output of SHG light from the SHG element 820 becomes unstable.

The method disclosed in Patent Document 2 is only a technique for stabilizing the light output in the continuous oscillation state, and does not consider the light output stabilization during modulation.

In view of the above problems, the optical wavelength conversion device of the present invention controls the DBR laser, the optical wavelength conversion element that receives the fundamental light emitted from the DBR laser, and outputs the second harmonic light, and the DBR laser. Including control means. Here, the DBR laser has an active region, a phase region, and a DBR region in which a distributed black reflector (DBR) is formed. The control means adjusts the current value of the drive current and the injection time so as to satisfy the following condition. That is, one condition is that when one cycle of the drive current is divided into two time regions having the same time, constant drive currents that are different in the two time regions are injected into either the active region or the DBR region. On the other hand, a binary drive current is injected in each of the two time regions. Another condition is that the sum of heat generated in one cycle by the drive current injected into the other region is approximately constant for each cycle. Here, the control means can be simplified as a configuration including an existing pulse width modulation circuit.

More specifically, the optical wavelength conversion device of the present invention controls the oscillation of the DBR laser as described above, the optical wavelength conversion element as described above, and the injection current to the DBR laser according to the modulation signal for each period. Control means for controlling the wavelength and light output is included. Then, one cycle of the drive current injected into the DBR laser is divided into the first time region t ₁ and the second time region t ₂ , and current injection is performed by the control means as follows. That is, in the first time domain t _1, one current I ₁ constant in the region of the active region and the DBR region, the other region, the current I ₃ at a certain time t _3, the first time domain The current I ₄ is injected at the remaining time t ₄ . Also, in the second time region t ₂ , the constant current I ₂ in one region, the current I ₅ in one region at a certain time t ₅ , and the current in the remaining time t ₆ in the second time region. I ₆ current is injected. Further, t ₁ = t ₂ and the control means controls the current of the drive current so that the sum of heat generated in one cycle by the drive current injected into the other region becomes substantially constant for each cycle. Adjust the value and injection time. In addition, the control means, when represented by the current _{I 3, I 4, I 5} , the other area of the driving voltage is respectively V ₃ when the _{I 6, V 4, V 5} , V 6, | I 3 × V ₃ −I ₄ × V ₄ | = | I ₅ × V ₅ −I ₆ × V ₆ | and adjust the drive current so that t ₃ + t ₅ is constant .

In the above configuration, while satisfying the following two conditions, the oscillation current and the optical output are controlled by controlling the injection current to the DBR laser according to the modulation signal, and the second harmonic light from the optical wavelength conversion element is controlled. The amount of light can be modulated according to the modulation signal. One is the condition for controlling the optical output mode of the DBR laser so that the optical energy in each period input to the optical wavelength conversion element is constant (this is the SHG light of the optical wavelength conversion element as shown in FIG. 2). This is a condition for not changing the wavelength dependency of the output). The other is to make the injection current vs. oscillation wavelength characteristics in the DBR region constant (that is, so that the relationship between the injection current into the DBR region and the oscillation wavelength of the DBR laser does not change), the active region, the phase region, and the DBR. This is a condition in which current is injected into each region in a manner in which the amount of generated heat is constant in each cycle. Note that “constant” in the description of the above configuration is used in a sense including not only when it is strictly constant, but also when it is approximately constant as long as it exhibits a certain degree of effect. In addition, the constant amount of generated heat means that the current is constant in a period within at least one period including the time when light of the latest wavelength is emitted from the DBR laser to the phase matching wavelength of the optical wavelength conversion element, and the injected current over one period. Means that the amount of heat generated by is constant for each period.

In the above structure, under the condition of t ₁ = t _2, the current I ₃ in order to satisfy the heat generation amount constant aspect, current I _4, current I _5, current I ₆ are each as shown in FIG. 1 (b) s It may be different, but it can also be binary (see Figure 3 and Figure 4).

Further, in view of the above problems, the optical wavelength conversion method of the present invention causes the fundamental wave light emitted from the DBR laser as described above to enter the optical wavelength conversion element and output the second harmonic light, and DBR according to the modulation signal. The oscillation current and the optical output are controlled by controlling the injection current to the laser for each period. Then, one cycle of the drive current injected into the DBR laser is divided into the first time region t ₁ and the second time region t ₂ , and current injection is performed as follows. That is, in the first time domain t _1, one current I ₁ constant in the region of the active region and the DBR region, the other region, the current I ₃ at a certain time t _3, the first time domain injecting current of the current I ₄ in the remainder of the time t _4. Also, in the second time region t ₂ , the constant current I ₂ in one region, the current I ₅ in one region at a certain time t ₅ , and the current in the remaining time t ₆ in the second time region. Inject I ₆ current. Further, the current value of the drive current and the injection are such that t ₁ = t ₂ and the sum of the amount of heat generated in one cycle by the drive current injected into the other region is approximately constant for each cycle. Adjust the time. In addition, if the driving voltages of the other region at currents I ₃ , I ₄ , I ₅ , and I ₆ are represented by V ₃ , V ₄ , V ₅ , and V ₆ , respectively, | I ₃ × V ₃ − I ₄ × V ₄ | = | I ₅ × V ₅ −I ₆ × V ₆ | and adjust the drive current so that t ₃ + t ₅ is constant.

Further, in view of the above problems, the method for controlling an optical wavelength conversion device of the present invention includes a first step, a second step, a third step, and a fourth step described below. The injection currents in the DBR region, phase region, and active region of the DBR laser are determined using the first to fourth steps. Here, in the first step, a condition is extracted in which the output of the second harmonic light from the optical wavelength conversion element during modulation or the conversion efficiency is substantially maximized. In the second step, a condition where mode hops are unlikely to occur with the DBR laser is extracted. In the third step, a condition for maximizing the output of the second harmonic light from the optical wavelength conversion element or the conversion efficiency is extracted. In the fourth step, a condition for making the output of the second harmonic light from the optical wavelength conversion element a desired value is extracted. Furthermore, a fifth step of setting a modulation current in the DBR region so as to realize a desired gradation may be included.

In view of the above problems, an image forming apparatus such as a laser display or a laser beam printer according to the present invention includes the light wavelength conversion device and at least one light scanning element. Here, an image is formed by scanning light emitted by the optical wavelength conversion device with an optical scanning element and adjusting the amount of second harmonic light according to the modulation signal or image data. Control using the control method of the optical wavelength conversion device may be performed.

According to the present invention, with a relatively simple control method, the thermal stability of the DBR laser and the optical wavelength conversion element can be ensured, and the optical wavelength conversion capable of stable high gradation control without depending on the modulation signal pattern. An apparatus or method, and a control method for the optical wavelength converter can be realized. In addition, an image forming apparatus capable of forming an image having a high-definition gradation expression can be realized by using the optical wavelength conversion apparatus or the control method according to the present invention. Further, when the control means is simplified as a configuration including an existing pulse width modulation circuit, the cost can be relatively low.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of an optical wavelength conversion apparatus or method according to the present invention will be described. The optical wavelength conversion device according to the present embodiment includes a DBR laser, an optical wavelength conversion element (SHG element) that receives the fundamental wave light emitted from the DBR laser and outputs the second harmonic light, and generates a modulated signal. A control means for controlling the DBR laser is included. The DBR laser has a DBR region in which an active region, a phase region, and a distributed Bragg reflector (DBR) are formed. Here, when one cycle of the drive current is divided into two time regions having the same time, a constant drive current different in each of the two time regions is applied to one of the active region and the DBR region. inject. Therefore, the sum of heat generated within one period by the drive current injected into this region is naturally constant for each period. In addition, a binary driving current is injected into each of the two time regions on the other side, and the sum of heat generated in one cycle by the driving current injected into the other region is approximately constant for each cycle. In this way, the current value of the drive current and the injection time are adjusted. Furthermore, the control means is simplified by a configuration including an existing pulse width modulation circuit.

When light having a recent wavelength is emitted from the DBR laser to the phase matching wavelength of the optical wavelength conversion element, a high amount of light (bright level) SHG light is emitted from the optical wavelength conversion element. The wavelength stability of the fundamental wavelength light from the DBR laser at this time is most required to ensure stable modulation and a sufficiently large extinction ratio, but the above configuration satisfies this requirement.

This will be described in more detail below. FIG. 1 (a) is a schematic diagram showing a configuration of an optical wavelength conversion device according to the present embodiment. In FIG. 1A, 101 is a semiconductor DBR laser, and 101a, 101b, and 101c are an active region, a phase region, and a DBR region, respectively. Further, 102 is a collimator lens, 103 is a condenser lens, 104 is an SHG element, 105 is a polarization inversion region, 106 is an optical waveguide, 107 is a wavelength filter, and 108 is SHG light. Further, 109 is a control means for controlling the injection current to the DBR laser 101 according to the modulation signal to control its oscillation wavelength and optical output, and 110 is a pulse width determination means. The control means 109 includes pulse width determining means 110 that converts input data into pulse width data and outputs the data.

FIG. 1 (b) schematically shows drive currents injected into the active region 101a and the DBR region 101c. One cycle T is divided into two time regions of time t, and currents I ₁ in the first time region and I _{2 in} the _second time region are injected into the active region. At this time, it is set so as to satisfy I ₁ > I ₂ .

On the other hand, for the DBR region, the current value and time of the drive current are controlled for each cycle in accordance with the modulation signal. Here, the current values in the DBR region in the first time region are I _3n and I _4n , and the current values in the DBR region in the second time region are I _5n and I _6n . The driving voltage of the DBR region at each current is V _3n , V _4n , V _5n , V _6n , and the time when each current is injected is t _3n , t _4n , t _5n , t _{6n, respectively.}
(N is an arbitrary number indicating the order of the period of the modulation signal). At this time, each current value and time is determined for each period so as to satisfy the following relationship.
I _3n × V _3n × t _3n + I _4n × V _4n × t _4n + I _5n × V _5n × t _5n + I _6n × V _6n × t _6n = constant (Formula 1)
| I _3n × V _3n −I _4n × V _4n | = | I _5n × V _5n −I _6n × V _6n | (Formula 2)
t _3n + t _5n = t (Formula 3)
If the relationship such as the above equation is satisfied, it becomes easy to control the drive current according to the modulation signal under the condition of t ₁ = t ₂ .

Here, control is performed so as to satisfy all of Formula 1, Formula 2, and Formula 3, but control is also possible so that only Formula 1 is satisfied.

At this time, the energy (I · V time integration) injected into the DBR region is constant for each period, so the amount of heat generated in one period in the DBR region is substantially constant regardless of the modulation signal. Of course, the injection current into the active region and the injection current into the phase region are also controlled so that the sum of the amounts of heat transferred from the active region 101a and the phase region 101b to the DBR region 101c is constant. Therefore, the relationship between the oscillation wavelength of the DBR laser 101 and the injection current value into the DBR region 101c can be made constant, and the oscillation wavelength can be stably controlled according to the modulation signal. As a result, light having a wavelength that is recent to the phase matching wavelength of the optical wavelength conversion element and light having a wavelength shifted from this wavelength can be input to the SHG element 104 with good controllability. At this time, even if the wavelength of the input light changes in each period, the light energy input to the SHG element 104 is constant, and the temperature of the SHG element 104 is kept constant. As a result, since the wavelength conversion efficiency of the SHG element 104 becomes constant, stable modulation of the SHG light 108 according to the modulation signal can be realized.

In this way, the control means 109 is such that the multi-value current value constituting the drive current or injection current is constant for each cycle, and the time for injecting the current is variable for each cycle according to the modulation signal. By controlling the injection time, the amount of second harmonic light for each period is adjusted.

In the above configuration, by changing the polarization inversion period and the element temperature of the optical wavelength conversion element 104 at any oscillation wavelength at the current value I ₃ , I ₄ , I ₅ , I ₆ of the DBR region 101c, It is possible to match the phase matching wavelength. However, in the case of I ₁ > I ₂ as shown in FIG. 1B, the output of the second harmonic light becomes the largest when phase matching is performed when the current is I ₃ or I ₄ . At this time, by adjusting the currents I ₃ and I ₄ , the oscillation wavelength of the DBR laser 101 is matched with or removed from the phase matching wavelength of the optical wavelength conversion element 104, and the light level and darkness of the second harmonic output are adjusted. The level ratio (extinction ratio) can be adjusted. Needless to say, a larger extinction ratio is desirable. An extinction ratio of 10 times or more is particularly effective for a display device such as a display. This is because the relationship between the injection current into the DBR region of the semiconductor laser and the oscillation wavelength is stable and the oscillation wavelength is modulated by the modulation signal with good controllability, and the temperature of the SHG element is stable and its phase matching. It is important that the wavelength dependence of the SHG optical output that defines the wavelength is stable. The configuration of the present invention satisfies these conditions. Further, by changing t _3n under the condition of Equation 3 by the pulse width determining means 110, the amount of the second harmonic light can be changed for each period. In this way, more specifically, the pulse width determining means 110 is used when a desired gradation is output in a certain cycle so that the second harmonic output can be adjusted at a desired maximum gradation. Determines a time width of t _{3 and} a time width of t ₅ according to a desired gradation value.

As described above, the control means 109, for example, changes the oscillation wavelength of the DBR laser when the current I ₃ is injected into the oscillation wavelength of the DBR laser when the currents I ₄ , I ₅ , and I ₆ are injected. Compared to the phase-matching wavelength of the optical wavelength conversion element. Then, the drive current to the active region, the injection current to the phase region, and the drive current to the DBR region are adjusted. At this time, the control means 109 is preferably configured such that the output of the second harmonic light when the current I ₃ is injected is the second harmonic light when the currents I ₄ , I ₅ , and I ₆ are injected. Each drive current is adjusted so that it is more than 10 times larger than the output.

The wavelength of the DBR laser can be arbitrarily selected as long as it can oscillate as a DBR laser and has a non-linear effect of the optical wavelength conversion element. Further, as the optical wavelength conversion element, a nonlinear optical crystal such as LiNbO ₃ (LN), KNbO ₃ (KN), KTiOPO ₄ (KTP), LiTaO ₃ (LT) can be used.

Embodiments of the present invention will be described below with reference to the drawings.

(Example 1)
FIG. 1 (a) shows a configuration diagram of Embodiment 1 according to the present invention. The semiconductor DBR laser 101 includes an active region 101a, a phase region 101b, and a DBR region 101c having a diffraction grating structure. The phase region 101b and the DBR region 101c can change the oscillation wavelength by changing the refractive index of the active layer by passing a current perpendicular to the pn junction.

The fundamental laser beam generated from the semiconductor DBR laser 101 by flowing current through the active region 101a, the phase region 101b, and the DBR region 101c by the control means 109 is transmitted through the collimator lens 102 and the condensing lens 103 to the light wavelength conversion element 104. It is condensed to. The optical wavelength conversion element 104 is a waveguide type quasi-phase matching element having an optical waveguide 106 made of lithium niobate (LiNbO ₃ ) having an element length of 12 mm and periodically formed with domain-inverted regions 105, and is a fundamental laser. The light can be converted to second harmonic light 108 of half wavelength. Since both the fundamental laser beam and the second harmonic light are output from the optical wavelength conversion element, the fundamental laser beam is cut using the wavelength filter 107.

Figure 2 shows the wavelength dependence of the second harmonic light output. In this optical wavelength conversion element, the phase matching wavelength is adjusted to 1064.0 nm by adjusting the polarization inversion period. The vertical axis in FIG. 2 is normalized with the maximum value of the second harmonic light output as 1.

In this embodiment, the control means 109 of the DBR laser 101 includes a pulse width modulation LSI (pulse width determination means) 110 corresponding to a clock of 50 MHz and 64 gradations, and is synchronized with an input clock having a period of 20 ns for 20 ns. The pulse width can be changed in steps of /64=312.5ps. This LSI is a common one used in laser beam printers.

A constant current of 10 mA is injected into the phase region 101b, and as shown in FIG. 3, the current of the active region 101a is set to I ₁ = 250 mA, I ₂ = 20 mA, and the pulse width t = 20 ns. Since I ₂ = 20 mA is a current not longer than the oscillation wavelength of the DBR laser used here, the optical output in the second time domain is zero. In addition, the current values of the DBR region 101c are I ₃ = I ₅ = ₆₀ mA and I ₄ = I ₆ = 20 mA. At this time, by maintaining t _3n + t _5n = 20 ns, the relationship of Equations 1 and 2 can be satisfied, and the amount of heat generated in the DBR region 101c becomes substantially constant. When the current in the DBR region increases, the refractive index of the DBR region decreases due to the carrier effect and the oscillation wavelength decreases. Conversely, when the current decreases, the refractive index of the DBR region increases and the oscillation wavelength increases. Therefore, the oscillation wavelength at this time was 1064.0 nm when I ₃ = 60 mA, and 1065.2 nm when I ₄ = 20 mA.

Here, the data string {64, 48, 16, 32, 0} is input, and the gradation output corresponding to each number is measured. The clock of the data string is 40 ns, twice that of the pulse width modulation LSI (pulse width determining means) 110. In the control means 109 of this embodiment, a data string {n, 64-n} is generated for the input data n, and a circuit for inputting to the pulse width modulation LSI is incorporated in the ASIC. Therefore, a data string {64, 0, 48, 16, 16, 48, 32, 32, 0, 64} is input to the pulse width modulation LSI with a clock of 20 ns. Here, the pulse width modulation LSI instructs the input of data N to output a pulse width of 20 ns ÷ 64 × N in a forward-aligned manner. Therefore, the drive current time of the DBR region 101c is (t _3n , t _5n ) = (20ns, 0ns), (15ns, 5ns), (5ns, 15ns), (10ns, 10ns), ( 0ns, 20ns). The second harmonic output at this time was {4.30, 3.20, 1.10, 2.10, 0.12} (mW), and an optical output substantially proportional to the pulse width could be obtained. The extinction ratio was 1:36.

Therefore, according to this embodiment, the optical output can be controlled by controlling the pulse width using the pulse width modulation LSI, that is, the SHG optical output of a short wavelength (532 nm) can be modulated according to the modulation signal. I understand that there is.

(Example 2)
In the present embodiment, an example in which the control means 109 of the DBR laser 101 generates a data string is different from that in the first embodiment. Since the configuration and set current value of the present embodiment are the same as those of the first embodiment, the description thereof is omitted.

Here, the data string {64, 48, 16, 32, 0} is input, and the gradation output corresponding to each number is measured. The clock of the data string is 40 ns, twice that of the pulse width modulation LSI (pulse width determining means) 110. In the control means 109 of this embodiment, for the input of data n, the pulse of 20 ns / 64 × n is first moved forward, and the pulse of 20 ns− (20 ns / 64 × n) is moved forward in the next 20 ns. Is output from the pulse width modulation LSI. Therefore, the drive current of the DBR region 101c has a waveform as shown in FIG. In the first time domain, the waveform is the same as that of the first embodiment, but in the second time domain, the time is inverted. The second harmonic output at this time was {4.29, 3.21, 1.12, 2.21, 0.13} (mW), and an optical output substantially proportional to the pulse width could be obtained. The extinction ratio was 1:33.

Here, the control unit 109, controlled so that the time width of the t ₃ is the time width of the position and the t ₅ to the front end of the first time region is located behind the end of the second time domain It is carried out. However, it is also possible to perform control so as to duration of the t ₃ is the time width of the t ₅ located behind the end of the first time region is located in front of the end of the second time domain is there.

Therefore, even in this embodiment, the optical output can be controlled by controlling the pulse width using the pulse width modulation LSI, that is, the SHG optical output with a short wavelength (532 nm) can be modulated according to the modulation signal. It turns out that it is.

Since the drive waveform of the present embodiment has fewer rising / falling times than the drive waveform of the first embodiment, this embodiment may be more advantageous when driving at a higher frequency.

On the other hand, it is also possible to fix the drive current waveform in the DBR region and perform pulse width modulation on the drive current in the active region. The advantage of this method is that the modulation frequency can be increased. It is known that the light intensity modulation time by current modulation in the active region is shorter than the wavelength modulation time by DBR current modulation, and the modulation frequency can be increased. In the present embodiment, an example with a frequency of 20 MHz is shown, but modulation of about 100 MHz is possible by pulse width modulation of the drive current in the active region.

In this case, for example, the currents I ₄ and I ₆ can be set to values equal to or less than the oscillation threshold of the DBR laser. The control means may, for example, the oscillation wavelength of the DBR laser while injecting the current I ₃ is the current as compared with the oscillation wavelength of the DBR laser while injecting I _5, the optical wavelength converting element The drive current or injection current can be adjusted so as to be close to the phase matching wavelength. In addition, in the control means, the output of the second harmonic light when the current I ₃ is injected is 10 times or more larger than the output of the second harmonic light when the current I ₅ is injected. Similarly, the drive current or injection current can be adjusted.

In the above embodiment, a constant current is injected into the phase region. However, a driving current synchronized with the driving current in the DBR region is input, and the equations 1, 2, and 3 are similarly satisfied in the phase region. It is also possible to set to. In this case, the laser oscillation condition of the resonator is accurately satisfied by adjusting the current value in the phase region, so that the laser oscillation spectrum is stabilized in a single mode, and a higher output second harmonic output can be obtained. . In this way, the control means injects a drive current synchronized with the drive current of the DBR region into the phase region, and drives the phase region so that the sum of heat generated within one cycle is approximately constant for each cycle. The current value of the current and the injection time can be adjusted.

From the embodiment described above, it can be seen that the second harmonic light output can be stably controlled by controlling the pulse width using the pulse width modulation LSI. This modulation method is pulse width modulation (PWM). In PWM, the multi-value current value that constitutes the drive current is constant for each period, the time for injecting the current is variable for each period according to the modulation signal, and the injection time is determined by the pulse width of the control means 109. The amount of second harmonic light for each period is adjusted by controlling the modulation circuit. According to this method, since only the pulse time width needs to be changed according to the modulation signal, the configuration of the modulation circuit of the control means 109 can be made relatively simple. In this way, the current value of the set one and other regions is maintained, and the current injection time of the other region is changed for each gradation while keeping t ₃ + t ₅ constant. The key can be expressed.

(Example 3)
FIG. 5 shows a configuration diagram of Embodiment 3 according to the present invention. In the present embodiment, a Peltier element 401 and a photodetector 402 are added to the configuration of FIG. The semiconductor DBR laser 101 is mounted on a substrate whose temperature can be adjusted by the Peltier element 401, and the temperature of the semiconductor DBR laser 101 can be controlled by controlling the current flowing through the Peltier element 401. Further, a photodetector 402 for measuring the second harmonic light 108 is provided, and the output of the photodetector 402 is input to the control means 109. Based on this measurement data, the drive currents of the active region 101a, phase region 101b, DBR region 101c, and Peltier element 401 of the semiconductor laser 101 are controlled.

In this configuration, the second harmonic output is adjusted according to the procedure shown in FIG. Detailed procedures of each step are shown in FIGS.

First, the first step: initial adjustment is performed according to the procedure shown in FIG. Referring to the initial setting values set in advance, as shown in FIG. 13, the current in the active region 101a is set to I ₁ = 500 mA, I ₂ = 35 mA, and the pulse width is set to t ₁ = t ₂ = 500 ns. Since I ₂ = 35 mA is a current equal to or less than the oscillation threshold of the DBR laser 101 used here, the light output in the _second time domain t ₂ is zero. A constant current of 25 mA is injected into the phase region 101b, and the current values of the DBR region 101c are I ₃ = ₆₀ mA and I ₆ = 10 mA. Here, t ₃ = t ₆ and t ₄ = t ₅ = 0 (see FIG. 13). At this time, by adjusting the temperature of the Peltier element 401 to 35 ° C., the output of the second harmonic light was approximately a maximum value of 27.6 mW. As described above, the first step includes a step of injecting a desired pulse width and a modulation current, which are temporarily set so as to maximize the gradation level, into the active region and the DBR region. In addition, a step of adjusting the injection current of the active region or the temperature of the DBR laser or the optical wavelength conversion element is included so that the output of the second harmonic light or the conversion efficiency at this time is substantially maximized. However, in this embodiment, the temperature of the DBR laser is adjusted.

Next, the second step: oscillation mode stabilization is performed according to the procedure shown in FIG. When I ₃ was changed up and down while the current value I ₆ = 10 mA in the DBR region 101c was fixed, the second harmonic output greatly decreased when exceeding 56 mA and 66 mA. Therefore, the current value I ₃ was set to 61.0 mA which is the average value. In this way, the second step detects the current value at which the output of the second harmonic light or the output of the fundamental wave light changes greatly when the modulation current in the DBR region is shifted back and forth from the temporary setting value, and the output A step of setting the modulation current to a current value with small fluctuations. However, in this embodiment, a current value at which the output of the second harmonic light changes greatly is detected.

Next, the third step: light output maximization is performed according to the procedure shown in FIG. In this step, under the conditions mode hopping does not occur, to adjust the current I _P of the current I ₃ and the phase region of the DBR region, to maximize the light output. In this way, the third step maintains the non-mode hop relationship and adjusts the injection current in the phase region and the modulation current in the DBR region to maximize the output or conversion efficiency of the second harmonic light. The step of setting the injection current and the modulation current in the DBR region is included. However, in this embodiment, the injection current in the phase region and the modulation current in the DBR region are set so that the output of the second harmonic light is maximized.

The conditions under which no mode hop occurs will be described.
The change Δλ _BRAGG of the Bragg wavelength (λ _BRAGG ) when the refractive index of the DBR region changes by Δn _D is expressed by the following equation.
Δλ _BRAGG / λ _BRAGG = Δn _D / n (Formula 4)
Here, the refractive index of each region of the DBR laser is expressed by n on the assumption that they are substantially equal.

The change Δλ _CAVITY of the resonator wavelength (λ _CAVITY ) when the refractive index of the DBR region changes by Δn _D and the refractive index of the phase region by Δn _P is expressed by the following equation.
Δλ _CAVITY / λ _CAVITY = (Δn _D · L _Deff + Δn _P · L _P ) / n (L _Deff + L _P + L _G ) (Formula 5)
Here, L _G is the length of the active region, L _P is the length of the phase region, and L _Deff is the effective length of the DBR region.

Since the condition that the mode hop does not occur is the case where the wavelength change rates in Equation 4 and Equation 5 are equal, solving the assumption that the right sides of Equation 4 and Equation 5 are equal, the following relationship is obtained.
(L _G + L _P ) Δn _D = L _P · Δn _P (Formula 6)

The amount of change in refractive index in each region is proportional to the amount of change per unit length of input power. Therefore, if the amount of change in input power in the DBR region and the phase region is ΔP _DBR and ΔP _Phase , respectively, Equation 6 can be rewritten as follows.
(L _G + L _P ) ΔP _DBR / L _D = ΔP _Phase (Formula 7)
Here, L _D is the length of the DBR region.

Therefore, continuous oscillation wavelength without causing mode hops by adjusting the current I _P of the current value I ₃ and the phase region of the DBR region so as to satisfy the formula 7 (a function of the DBR modulation current vs phase current) Can be changed. The input power amount in each region can be calculated by grasping the relationship between current and voltage in advance. That is, the relationship not to hop the mode is obtained in advance. In this step it is adjusted so that the light output of the SHG light is maximized while maintaining the relationship of Equation 7, setting the current I _P of the current value I ₃ and the phase region of the DBR region.

As a result of adjusting so that the optical output of SHG light was maximized while maintaining the relationship of Equation 7, I _p = 42.7 mA, I ₃ = 62.0 mA, and the second harmonic output was 29.0 mW.

Next, the fourth step: light output adjustment is performed according to the procedure shown in FIG. Here, in order to match the desired optical output, the modulation current in the active region and the current value in the phase region are determined without changing the oscillation wavelength. In order to change only the DBR laser output without changing the oscillation wavelength, the resonator wavelength should be kept constant. That is, the following relationship may be satisfied.
Δn _G · L _G + Δn _P · L _P = 0 (Formula 8)
Here, Δn _G is the amount of change in the refractive index of the active region.

The amount of change in refractive index in each region is proportional to the amount of change per unit length of input power. Therefore, if the change amount of the input power in the active region is ΔP _Gain , Equation 8 can be rewritten as follows.
ΔP _Gain + ΔP _Phase = 0 (Formula 9)

Accordingly, while maintaining the oscillation wavelength by adjusting the current I _P of the current I _G and the phase region of the active region so as to satisfy equation 9 (a function of the active region current vs phase current), the light output of the DBR laser Can be changed. The input power amount in each region can be calculated by grasping the relationship between current and voltage in advance. That is, the relationship in which the resonator wavelength does not change is obtained in advance. In this step is adjusted so that the light output of the SHG light while maintaining the relationship of equation 9 becomes a desired value to set the current I _P of the current I _G and the phase region of the active region. The desired value of the light output of the SHG light is required in relation to the light output intensity of other colors such as red and blue when, for example, full color image display is performed. Adjustments were made under these conditions to obtain a second harmonic output of 30.0 mW. In this way, the fourth step is to control the injection current in the active region and the injection current in the phase region while maintaining the relationship that the resonator wavelength of the DBR laser does not change, and the output of the second harmonic light is the target value. The step of setting the injection current in the active region and the injection current in the phase region is included.

The light output adjustment is now complete. As a result of performing the above steps, I ₁ = 505 mA and I ₂ = 35 mA.

Gradation output is performed while maintaining the second harmonic light output as the initial output. First, the pulse pattern is matched with the pattern of the maximum gradation, the light output is detected by the photodetector 402, and it is determined whether or not it is within 5% of the initial output. At this time, since the light output fluctuation was within 5% of the initial output, no adjustment was made (fifth step). The procedure for this step is shown in FIG.

Next, as shown in FIG. 14, as in the first embodiment, the gradation data string {64, 48, 16, 32, 0} is input to the control means 109, and the second harmonic corresponding to each gradation data is input. Wave output was measured. The second harmonic output at this time was {29.2, 21.0, 7.5, 15.2, 0.8} (mW), and an optical output substantially proportional to the pulse width could be obtained (sixth step). The procedure for this step is shown in FIG.

Returning again to the fifth step shown in FIG. 11 and checking the light output, it was reduced to 27.5 mW, so the control returns to the fourth step for adjustment. When adjusted again according to the relationship of Equation 9, the second harmonic light output was 29.8 mW. From the above, it was found that the light output can be adjusted.

In this way, the light quantity can be stabilized by adding a process for controlling the light quantity at all times or at regular intervals. For this purpose, for example, the amount of light at the maximum gradation is monitored. And when the value fluctuates greatly compared to the predetermined value, for example, when there is a fluctuation of 5% or more, the light amount becomes the predetermined value by performing the fourth step and the fifth step again. Adjust to. If this operation cannot be adjusted to the predetermined value, the third to fifth steps are performed again. Further, when the adjustment is impossible, the light amount is adjusted in the order of priority from the second process to the fifth process and from the first process to the fifth process (see FIG. 11). As a result, even when there is a change in the amount of SHG light due to a change in environmental temperature or the like, it can be controlled to a desired value.

In this embodiment, the temperature adjustment of the DBR laser is performed by the Peltier element 401. However, it is also possible to adjust the heater electrode by integrating the heater electrode in the DBR laser 101 (that is, the temperature adjustment means is a DBR). Consisting of a heater integrated in the laser). Even if there is no mechanism for adjusting the temperature of the DBR laser, it is also possible to adjust with the injection current of the active region in the first step.

In addition, although the relationship that does not cause mode hopping is expressed by Equation 7, in addition to this method, considering that the heat generation amount is proportional to the square of the current, the oscillation wavelength is a quadratic function of I _p and I _3. The same effect can be obtained by a method such as

In this embodiment, the photodetector 402 that monitors the light amount of the second harmonic light is used. However, another light detector that monitors the light amount of the fundamental wave light of the DBR laser may be provided. In that case, since the conversion efficiency to the second harmonic light can be grasped, in the above process, the conversion efficiency can be maximized or maximized, so that the adjustment can be performed with higher accuracy.

(Example 4)
Example 4 relates to an image display device using an optical wavelength conversion device according to the present invention. FIG. 15 shows a schematic configuration diagram of the image display apparatus of the present embodiment. In this image display device, the laser light output from the light wavelength conversion device 301 that emits the green modulated light, the modulated light source 302 that emits the red laser, and the modulated light source 303 that emits the blue laser described in the above embodiment is transmitted by the dichroic mirror 304. Combined. The combined laser beam is scanned by the horizontal scanning element 305 and the vertical scanning element 306 to form a scanning line on the screen 307. A two-dimensional full-color image is displayed on the screen 307 by modulating the output of each light source 301, 302, 303 with the gradation information of each color of red, green, and blue generated from the full-color image information. Here, the modulation current may be controlled for each pixel period in accordance with the modulation signal corresponding to the pixel of the image on the screen 307.

Further, when the control method as described in the third embodiment is used, in the optical image forming apparatus of the present embodiment, it is possible to reduce the light output fluctuation due to the environmental temperature change or the like. Specifically, when the power is turned on, the first to fourth steps are performed to determine the initial light output, and the fifth step is performed during a time period when the image is not drawn to keep the light output constant. Can be kept in. That is, when the apparatus is started up, the first to fourth steps are performed, or the second harmonic light output is adjusted by the steps shown in FIG. 11 within the time when the image is not drawn.

Since the optical wavelength conversion device according to the present invention has a modulation performance equivalent to that of a red semiconductor laser or the like, the image display device can display an image having high-definition gradation expression. The optical wavelength conversion apparatus or method of the present invention can be used for an image forming apparatus such as a laser beam printer and a copying machine in addition to the laser display.

It is a figure explaining typical composition and operation of an optical wavelength converter of an embodiment and an example of the present invention. FIG. 5 is a diagram showing the relationship between the wavelength of the SHG element of Example 1 of the present invention and the second harmonic output. It is a figure explaining the drive current waveform and 2nd harmonic output of Example 1 of this invention. It is a figure explaining the drive current waveform and 2nd harmonic output of Example 2 of this invention. FIG. 6 is a diagram showing a configuration of Example 3 of the present invention. FIG. 5 is a diagram illustrating an outline of a control method according to a third embodiment of the present invention. FIG. 10 is a diagram illustrating a first step of a control method according to Example 3 of the present invention. FIG. 10 is a diagram illustrating a second step in the control method according to the third embodiment of the present invention. FIG. 10 is a diagram illustrating a third step in the control method according to the third embodiment of the present invention. FIG. 10 is a diagram illustrating a fourth step in the control method according to the third embodiment of the present invention. FIG. 10 is a diagram illustrating a fifth step of the control method according to the third embodiment of the present invention. FIG. 10 is a diagram illustrating a sixth step in the control method according to the third embodiment of the present invention. FIG. 10 is a diagram illustrating current waveforms injected into an active region and a DBR region in the first step of the control method according to Example 3 of the present invention. FIG. 10 is a diagram showing a current waveform injected into an active region and a DBR region in the sixth step of the control method of Example 3 of the present invention, and a second harmonic light output waveform at that time. FIG. 9 is a schematic diagram of an image display device according to Example 4 of the present invention. It is a schematic diagram which shows the structure of a prior art example.

Explanation of symbols

101… DBR laser
101a ... Active region of DBR laser
101b… DBR laser phase region
101c… DBR region of DBR laser
104 ... Optical wavelength conversion element (SHG element)
108… Second harmonic light (SHG light)
109 ... Semiconductor DBR laser control means
110 ... Pulse width determination means (pulse width modulation LSI)
301 ... Optical wavelength converter of the present invention (green modulated light source)
302… Red modulated light source
303… Blue modulated light source
305 ... Optical scanning element (horizontal scanning element)
306 ... Optical scanning element (vertical scanning element)
307… screen
401… Temperature adjustment means (Peltier element)
402… Photodetector

Claims (12)

A DBR laser having an active region, a phase region, and a DBR region in which a distributed black reflector (DBR) is formed, and an optical wavelength conversion element that receives the fundamental light emitted from the DBR laser and outputs the second harmonic light And an optical wavelength converter including control means for controlling the oscillation wavelength and the optical output by controlling the injection current to the DBR laser according to the modulation signal for each period,
One period of drive current injected into the DBR laser is divided into a _first time region t ₁ and a second time region t ₂ ,
In the first time domain t ₁
One of the active region and the DBR region has a constant current I ₁ , the other region has a current I _{3 at} a certain time t ₃ , and a current I ₃ at the remaining time t ₄ of the first time region. ₄ currents are injected,
In the second time domain t _2,
A constant current I ₂ is injected into the one region, a current I ₅ is injected into the other region at a certain time t ₅ , and a current I ₆ is injected at the remaining time t ₆ of the second time region,
t ₁ = t ₂ and the control means controls the current value of the drive current so that the sum of heat generated in one cycle by the drive current injected into the other region is constant for each cycle. And adjust the injection time ,
Further, the control means may be configured such that when the driving voltages of the other regions at the currents I ₃ , I ₄ , I ₅ , I ₆ are represented by V ₃ , V ₄ , V ₅ , V ₆ , I ₃ × V ₃ −I ₄ × V ₄ | = | I ₅ × V ₅ −I ₆ × V ₆ | and the drive current is adjusted so that t ₃ + t ₅ is constant. Conversion device. 2. The optical wavelength converter according to claim 1, wherein the control means includes pulse width determining means for converting input data into pulse width data and outputting the converted data. The control means, the _{I 3 × V 3 × t 3} + I 4 × V 4 × t 4 + I 5 × V 5 × t 5 + I 6 × V 6 × t 6 to adjust the drive current as is constant The optical wavelength converter according to claim 1 or 2, wherein The one region is an active region, the optical wavelength conversion device according to any one of claims 1 to 3 wherein the other region is characterized in that it is a DBR region. The control means is configured such that the oscillation wavelength of the DBR laser when the current I ₃ is injected is compared with the oscillation wavelength of the DBR laser when the currents I ₄ , I ₅ , and I ₆ are injected. The drive current to the active region, the injection current to the phase region, and the drive current to the DBR region are adjusted so as to be close to the phase matching wavelength of the optical wavelength conversion element. 4. The optical wavelength converter according to 4 . The one region is DBR region, the optical wavelength conversion device according to any one of claims 1 to 3, wherein the other region is the active region. The control means is configured such that the oscillation wavelength of the DBR laser when the current I ₃ is injected is greater than the oscillation wavelength of the DBR laser when the current I ₅ is injected. The optical wavelength according to claim 6 , wherein the drive current to the active region, the injection current to the phase region, and the drive current to the DBR region are adjusted so as to be close to the phase matching wavelength of Conversion device. Optical wavelength conversion device according to any one of claims 1 to 7, characterized in that the first photodetector to monitor the output of the second harmonic light is provided. 9. The optical wavelength converter according to claim 8 , wherein a second photodetector for monitoring the fundamental light output is provided. Optical wavelength conversion device according to any one of claims 1 to 9, characterized in that the temperature adjusting means is provided on at least one of the DBR laser and the light wavelength converting element. The fundamental wave light emitted from the DBR laser having the active region, the phase region, and the DBR region where the distributed Bragg reflector (DBR) is formed is incident on the optical wavelength conversion element, and the second harmonic light is output to the modulated signal. In response to this, an optical wavelength conversion method for controlling the oscillation wavelength and optical output by controlling the injection current to the DBR laser for each period,
One period of drive current injected into the DBR laser is divided into a _first time region t ₁ and a second time region t ₂ ,
In the first time domain t ₁
One of the active region and the DBR region has a constant current I ₁ , the other region has a current I _{3 at} a certain time t ₃ , and a current I ₃ at the remaining time t ₄ of the first time region. Inject ₄ currents,
In the second time domain t _2,
A constant current I ₂ is injected into the one region, a current I ₅ is injected into the other region at a certain time t ₅ , and a current I ₆ is injected at the remaining time t ₆ of the second time region,
t ₁ = t ₂ and the current value and injection time of the drive current are adjusted so that the sum of the heat generated in one cycle by the drive current injected into the other region is constant for each cycle. And
Further, when the driving voltages of the other region at currents I ₃ , I ₄ , I ₅ , and I ₆ are represented by V ₃ , V ₄ , V ₅ , and V ₆ , respectively, | I ₃ × V ₃ − An optical wavelength conversion method characterized by adjusting the drive current so that I ₄ × V ₄ | = | I ₅ × V ₅ −I ₆ × V ₆ | and t ₃ + t ₅ is constant . Claim 1 has an optical wavelength conversion device and the at least one optical scanning device according to any one of 10 to scan the light emitted by the light wavelength conversion device in the optical scanning device, and according to the modulation signal An image is formed by adjusting the amount of the second harmonic light.

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