DESCRIPTION SHORT-PULSE LIGHT SOURCE, LASER LIGHT EMISSION METHOD, OPTICAL DEVICE, OPTICAL DISC DEVICE, AND OPTICAL PICKUP Technical Field The present invention relates to a short-pulse light source, a laser light emission method, an optical device, an optical disc device, and an optical pickup. For example, the present invention is preferably applied to an optical disc device in which information is recorded using light beams. Background Art Conventionally, disc-shaped optical information recording media have been widely used as optical information recording media, and CDs (Compact Discs), DVDs (Digital Versatile Discs), Blu-ray Discs (registered trademark, hereinafter called BDs), and the like are generally used. On the other hand, in an optical disc device compatible with such optical information recording media, various types of content, such as music content and video content, or various types of information, such as various data for a computer, is recorded on an optical information recording medium. Particularly, in recent years, the amount of information has been increasing due to higher definition of images and higher sound quality of music, and also an increase in the number of pieces of content to be recorded on an optical information recording medium has been demanded. Thus, higher capacity has been demanded for optical information recording media. Accordingly, an optical information recording medium has been suggested that is made by using a material for forming recording pits by causing a two-photon absorption reaction using light as a method for increasing the capacity of the optical information recording medium, whereby information is three-dimensionally recorded in the thickness direction of the optical information recording medium (e.g., see Patent Document 1). Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-37658 Incidentally, the two-photon absorption reaction is a phenomenon that occurs only using light of high intensity, and thus a light source achieving a high emission intensity needs to be used as a light source. As the light source, a short-pulse light source that outputs laser light in a short-pulsed manner, such as a so-called picosecond laser or femtosecond laser, is used. For example, a titanium sapphire laser and a YAG (yttrium aluminum garnet) laser are known. However, in such a short-pulse light source, output of short pulses is realized by an operation of an optical component provided outside a light generator. Therefore, the short-pulse light source typically has a large size and a high price, and thus mounting it in an optical disc device is unpractical. Here, if laser light can be output in a pulsed manner directly from a semiconductor laser, which is a compact light generator typically used in an optical disc device, it is unnecessary to provide an optical component outside the light generator, and the size of the short-pulse light source can be significantly reduced. When it is assumed that laser light can be output in a short-pulsed manner from a semiconductor laser, the semiconductor laser needs to be controlled so that laser light is output in a desired short-pulsed manner in accordance with application of voltage. Disclosure of Invention The present invention has been made in view of the above-described points and is directed to providing a short-pulse light source and a laser light emission method that are capable of controlling output of pulses from a semiconductor laser, and an optical device, an optical disc device, and an optical pickup that use the short-pulse light source. In order to solve the above-described problems, a short-pulse light source and an optical device according to the present invention are provided with a semiconductor laser configured to emit laser light and a laser control unit configured to control a pulse width of a pulse-shaped drive voltage pulse when causing specific output light having a pulse-shaped specific peak and a specific slope to be emitted as the laser light, an emission intensity of the specific slope being lower than an emission intensity of the specific peak, by applying a laser drive voltage having the drive voltage pulse to the semiconductor laser, thereby adjusting a ratio between the specific peak and the specific slope. Accordingly, in the present invention, specific output light having a specific peak and a specific slope at an arbitrary ratio can be output as laser light. Also, in a laser light emission method according to the present invention, a pulse width of a pulse-shaped drive voltage pulse is controlled when specific output light having a pulse-shaped specific peak and a specific slope is caused to be emitted as laser light, an emission intensity of the specific slope being lower than an emission intensity of the specific peak, by applying a laser drive voltage having the drive voltage pulse to a semiconductor laser, whereby a ratio between the specific peak and the specific slope is adjusted. Accordingly, in the present invention, specific output light having a specific peak and a specific slope at an arbitrary ratio can be output as laser light. Furthermore, an optical disc device and an optical pickup according to the present invention are provided with a semiconductor laser configured to emit laser light, an objective lens configured to irradiate an optical information recording medium with the laser light, and a laser control unit configured to control a pulse width of a pulse-shaped drive voltage pulse when causing specific output light having a pulse-shaped specific peak and a specific slope to be emitted as the laser light, an emission intensity of the specific slope being lower than an emission intensity of the specific peak, by applying a laser drive voltage having the drive voltage pulse to the semiconductor laser, thereby adjusting a ratio between the specific peak and the specific slope. Accordingly, in the present invention, specific output light having a specific peak and a specific slope at an arbitrary ratio can be output as laser light. Also, a short-pulse light source according to the present invention is provided with a semiconductor laser configured to emit laser light having a first wavelength by being applied with a laser drive voltage that is lower than an oscillation voltage value for causing relaxation oscillation, and a laser control unit configured to control a rise of a pulse-shaped drive voltage pulse when applying a laser drive voltage having the drive voltage pulse to the semiconductor laser, thereby switching laser light emitted from the semiconductor laser between oscillation output light generated by relaxation oscillation and specific output light having a wavelength in the vicinity of the first wavelength and a second wavelength shorter than the first wavelength. Accordingly, in the present invention, switching between oscillation output light and specific output light can be freely performed using a rise of a drive voltage pulse. Furthermore, an optical disc device according to the present invention is provided with a semiconductor laser configured to emit laser light having a first wavelength by being applied with a laser drive voltage that is lower than an oscillation voltage value for causing relaxation oscillation, an irradiation unit configured to irradiate an optical information recording medium with the laser light, and a laser control unit configured to control a rise of a pulse-shaped drive voltage pulse when applying a laser drive voltage having the drive voltage pulse to the semiconductor laser, thereby causing oscillation output light generated by relaxation oscillation to be emitted as the laser light during a reproduction process and causing specific output light having a wavelength in the vicinity of the first wavelength and a second wavelength shorter than the first wavelength to be emitted as the laser light during a recording process. Accordingly, in the present invention, switching between oscillation output light and specific output light can be freely performed using a rise of a drive voltage pulse. According to the present invention, specific output light having a specific peak and a specific slope at an arbitrary ratio can be output as laser light, which realizes a short-pulse light source and a laser light emission method that are capable of controlling output of pulses from a semiconductor laser, and an optical device, an optical disc device, and an optical pickup that use the short-pulse light source. Brief Description of Drawings Fig. 1 is a schematic diagram illustrating a configuration of a short-pulse light source according to a first embodiment. Fig. 2 is a schematic diagram illustrating a pulse signal and a laser drive voltage. Fig. 3 is a schematic diagram for explaining a relationship between injection carrier density and photon density. Fig. 4 is a schematic diagram for explaining a relationship between injection carrier density and carrier density. Fig. 5 is a schematic diagram for explaining a relationship between injection carrier density and photon density. Fig. 6 is a schematic diagram for explaining photon density at PTl. Fig. 7 is a schematic diagram for explaining photon density at PT2. Fig. 8 is a schematic diagram for explaining photon density at PT3. Fig. 9 is a schematic diagram illustrating an actual waveform of emitted light. Fig. 10 is a schematic diagram for explaining a drive current and an emission intensity. Fig. 11 is a schematic diagram illustrating a configuration of a light measurement device. Fig. 12 is a schematic diagram illustrating the shapes of individual pulses. Fig. 13 is a schematic diagram illustrating a relationship between a pulse signal and a drive voltage pulse. Fig. 14 is a schematic diagram illustrating voltages and waveforms of laser light. Fig. 15 is a schematic diagram illustrating laser light when voltage is 8.8 [V]. Fig. 16 is a schematic diagram illustrating laser light when voltage is 13.2 [V]. Fig. 17 is a schematic diagram illustrating laser light when voltage is 15.6 [V]. Fig. 18 is a schematic diagram illustrating laser light when voltage is 17.8 [V]. Fig. 19 is a schematic diagram illustrating laser light when voltage is 38.4 [V]. Fig.. 20 is a schematic diagram for explaining an effect of a BPF. Fig. 21 is a schematic diagram for explaining an effect of a BPF. Fig. 22 is a schematic diagram illustrating a waveform of specific output light. Fig. 23 is a schematic diagram illustrating changes of a pulse width and a specific slope. Fig. 24 is a schematic diagram for explaining control of specific output light using a pulse width. Fig. 25 is a schematic diagram for explaining a relationship between a pulse width and a voltage for shifting to a specific mode. Fig. 26 is a schematic diagram for explaining a rising slope. Fig. 27 is a schematic diagram for explaining a rising slope and waveforms of laser light. Fig. 28 is a schematic diagram for explaining control of a mode using a rising slope. Fig. 29 is a schematic diagram illustrating a configuration of an optical disc. Fig. 30 is a schematic diagram illustrating an entire configuration of an optical disc device. Fig. 31 is a schematic diagram illustrating a configuration of an optical pickup. Fig. 32 is a schematic diagram illustrating an optical path of a servo light beam. Fig. 33 is a schematic diagram illustrating an optical path of an information light beam. Fig. 34 is a schematic diagram for explaining comparison of laser drive voltages in a recording process. Fig. 35 is a schematic diagram illustrating a set pulse and a waveform of laser light in a recording process. Fig. 36 is a schematic diagram for explaining a positional deviation of a focal point due to a specific peak and a specific slope. Fig. 37 is a schematic diagram for explaining comparison of laser drive voltages in a reproduction process. Fig. 38 is a schematic diagram for explaining a set pulse and a waveform of laser light in a reproduction process. Best Modes for Carrying Out the Invention Hereinafter, an embodiment of the present invention will be described in detail in the following order with reference to the drawings. 1. First embodiment (control of drive voltage of short-pulse light source) 2. Second embodiment (application of short-pulse light source to optical disc device) (1) First embodiment (1-1) Configuration of short-pulse light source In Fig. 1, reference numeral 1 denotes an entire short-pulse light source according to this embodiment. The short-pulse light source 1 includes a laser control unit 2 and a semiconductor laser 3. The semiconductor laser 3 is a typical semiconductor laser that uses semiconductor emission (e.g., SLD3233 made by Sony Corporation). The semiconductor laser 3 is configured to output laser light LL in a pulsed manner through a drive voltage control process (details will be described below) performed by the laser control unit 2. The laser control unit 2 includes a pulse generator 4 and an LD (Laser Diode) driver 5. As illustrated in part (A) of Fig. 2, the pulse generator 4 generates a pulse signal SL in which pulse-shaped generated signal pulses SLw are discretely generated, and supplies the pulse signal SL to the LD driver 5. At this time, the pulse generator 4 controls the signal level of the generated signal pulses SLw in accordance with control of an external device, for example. As illustrated in part (B) of Fig. 2, the LD driver 5 amplifies the pulse signal SL with a predetermined amplification factor to generate a laser drive voltage DJ in which drive voltage pulses DJw are generated in accordance with the generated signal pulses SLw, and supplies the laser drive voltage DJ to the semiconductor laser 3. At this time, the voltage value of the drive voltage pulses DJw is determined in accordance with the signal level of the generated signal pulses SLw. Then, the semiconductor laser 3 outputs laser light LL in a pulsed manner in accordance with the laser drive voltage DJ. As described above, the short-pulse light source 1 is configured to output laser light LL in a pulsed manner directly from the semiconductor laser 3 in accordance with control performed by the laser control unit 2. (1-2) Output of laser light in pulsed manner in relaxation oscillation mode The following equation is a so-called rate equation that expresses characteristics of laser. Here,  represents a confinement factor, τPh represents a photon lifetime, τs represents a carrier lifetime, Cs represents a spontaneous emission coupling coefficient, d represents the thickness of an active layer, q represents an elementary charge, gmax represents a maximum gain, N represents a carrier density, S represents a photon density, J represents an injection carrier density, c represents the velocity of light, N0 represents a clearing carrier density, and ng represents a group index. (Formula Removed) Figs. 3 and 4 illustrate relationships between the photon density S and the carrier density N and the injection carrier density J obtained from equation (1). Note that, in Figs. 3 and 4, calculation was performed under the assumption that  = 0.3, Ag = 3e"16 [cm2], τph = le-12 [s] , τS = le-9 [s], C, = 0.03, d = 0.1 [µm], and q = 1.6e-19 [C] . As illustrated in Fig. 4, a typical semiconductor laser starts emitting light at a pre-saturation point S1, at which the carrier density N is just lower than that of a saturation state in accordance with an increase in the injection carrier density J (that is, laser drive voltage DJ) . Also, as illustrated in Fig. 3, the semiconductor laser increases the photon density S (that is, emission intensity) in accordance with an increase in the injection carrier density J. Also, as illustrated in Fig. 5, which corresponds to Fig. 3, it can be understood that the photon density S further increases in accordance with a further increase in the injection carrier density J. In Figs. 5, 6, and 7, the horizontal axis indicates the time from when application of a laser drive voltage DJ (that is, injection carrier density J) starts at points PT1, PT2, and PT3 illustrated in Fig. 5, and the vertical axis indicates the photon density S. As illustrated in Fig. 6, it was determined that, at point PT1 indicating a case where the highest laser drive voltage DJ is applied, the amplitude of the photon density S increases due to significant oscillation of relaxation oscillation and an oscillation period ta serving as a period of amplitude (that is, from a minimum value to the minimum value) is short of about 60 [ps]. The value of the photon density S is the largest at the amplitude of a first wave that appears just after start of emission, gradually attenuates in a second wave, a third wave, and so on, and eventually stabilizes. The maximum value of the first wave in the photon density S at point PT1 was about 3xl016, about three times a stable value (about lxl016), which is a value when the photon density S is stabilized. Here, an emission start time τd from when application of a laser drive voltage DJ starts to when emission of light starts can be calculated using the rate equation in equation (1) . That is, when it is assumed that the photon density S = 0 because oscillation has not started, the upper equation in equation (1) can be expressed by the following equation. (Equation Removed) Here, when it is assumed that the carrier density N is at a threshold value Nth the emission start time xd can be expressed by the following equation. (Equation Removed) That is, it can be understood that the emission start time τd is inversely proportional to the injection carrier density J. At point PTl illustrated in Fig. 6, the emission start time τd is calculated as about 200 [ps] in accordance with equation (3). At point PTl, a laser drive voltage DJ having a large voltage value is applied, and thus the emission start time xd is short. As illustrated in Fig. 7, at point PT2 where the value of the applied laser drive voltage DJ is smaller than at point PTl, definite relaxation oscillation occurred, but the amplitude of oscillation was smaller than at point PTl and the oscillation period ta was longer of about 100 [ps]. Also, at point PT2, the emission start time xd was about 400 [ps], longer than at point PTl. The maximum value of a first wave in the photon density S at point PT2 was about 8xl015, about twice a stable value (about 4xl015) . As illustrated in Fig. 8, it was determined that, at point PT3 where the value of the supplied laser drive voltage DJ is smaller than at point PT2, relaxation oscillation hardly occurs and the emission start time τd is relatively long of about 1 [ns]. The maximum value in the photon density S at point PT3 was almost the same as a stable value, about 1.2xl015. In a typical laser light source, a difference in emission intensity just after start of emission is intentionally decreased by applying, to a semiconductor laser, a relatively low laser drive voltage DJ that satisfies a condition (voltage value) in which relaxation oscillation hardly occurs as at point PT3, whereby laser light LL is stably output. Hereinafter, a mode in which the semiconductor laser 3 outputs laser light LL using a low voltage that does not cause relaxation oscillation is called a normal mode, and the laser light LL output in the normal mode is called normal output light LNp. However, in the short-pulse light source 1 according to this embodiment, relaxation oscillation is caused as at points PT1 and PT2, whereby an instantaneous maximum value of the emission intensity of laser light is increased to higher than a stable value (e.g., 1.5 times or more). Also, a large value can be selected as a voltage value for causing relaxation oscillation (hereinafter this is called an oscillation voltage value a), and thus laser light having a high emission intensity according to a large oscillation voltage value a can be emitted. That is, the emission intensity of laser light can be significantly increased compared to in a related art by applying a laser drive voltage DJ having an oscillation voltage value a to the same semiconductor laser. For example, at point PT1, the photon density S of the first wave of relaxation oscillation is about 3xl016, and the emission intensity of the semiconductor laser 3 can be increased by twenty times or more compared to at point PT3 (about 1.2xl015) indicating a case where a conventional voltage value is applied. Fig. 9 illustrates the emission intensity that was actually measured when a relatively high laser drive voltage DJ was applied to a typical semiconductor laser (SLD3233VF, made by Sony Corporation). It was determined from the figure that the relaxation oscillation that is seen in the photon density S in Figs. 6 and 7 is represented as is in the emission intensity and that similar relaxation oscillation actually occurs as the emission intensity. Note that Fig. 9 illustrates the waveform of laser light LL that was obtained when a laser drive voltage DJ was supplied to the semiconductor laser in a rectangular pulsed manner. Note that a portion supplied in a pulsed manner of the laser drive voltage DJ is hereinafter called a drive voltage pulse DJw. Part (A) of Fig. 10 is a figure corresponding to Fig. 7. For example, as illustrated in part (B) of Fig. 10, the laser control unit 2 of the short-pulse light source 1 supplies, to the semiconductor laser 3, a laser drive voltage DJ having an oscillation voltage value α1 that is sufficient to cause relaxation oscillation, as a drive voltage pulse DJw. At this time, the laser control unit 2 applies the laser drive voltage DJ having a rectangular pulse as the drive voltage pulse DJw for the time of the sum of the emission start time xd and the oscillation period ta (xd+ta). Hereinafter, this time is called a current wave supplying time. Accordingly, as illustrated in part (C) of Fig. 10, the laser control unit 2 can cause the semiconductor laser 3 to emit only a first wave based on relaxation oscillation and can cause the semiconductor laser 3 to emit pulse-shaped laser light LL having a high emission intensity (hereinafter this is called oscillation output light LMp). Also, by supplying a pulse-shaped drive voltage pulse DJw, the laser control unit 2 can shorten the time of applying a laser drive voltage DJ having a high voltage value, and can suppress a trouble of the semiconductor laser 3 that occurs due to overheat or the like of the semiconductor laser 3. On the other hand, as illustrated in part (D) of Fig. 10, the laser control unit 2 supplies, to the semiconductor laser 3, a drive voltage pulse DJw having an oscillation voltage value α2, which is sufficient to cause relaxation oscillation and which is smaller than the oscillation voltage value