JP2005197511A - Optical pulse generator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pulse generator with which shortened optical pulses can be stably obtained. <P>SOLUTION: An n-type buffer layer, an n-type clad layer, an active layer 38, a p-type clad layer and a p-type cap layer are sequentially laminated on an n-type substrate, and an (n) electrodes 44 is provided on a lower surface of the substrate. Furthermore, a stripe-like projection 30 formed in the cap layer is divided into three parts, a (p) electrode 42 is provided on a central projecting portion 30, (p) electrodes 42 are provided on both side projecting portions 30, a pulse current is applied to the (p) electrode 42 as a gain region, and an inverse bias voltage is applied to each (p) electrode 43 as a saturable absorption region, thereby constituting a semiconductor laser 10. A portion of an optical pulse outputted from the semiconductor laser 10 is then branched by an optical branching filter 25, and fed back into a laser light resonator after imparting a delay time matched with a timing interval of optical pulse streams by a feedback optical system 21. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical pulse generating element that generates ultrashort pulsed light using a semiconductor laser.

  Semiconductor lasers that are small, light and capable of direct modulation are widely used as ultrashort pulse light sources.

  Regarding a method for obtaining femtosecond order pulse emission in a semiconductor laser, for example, Optics Vol. 29, No. 8, p.495 (Non-Patent Document 1) reported pulse emission with a pulse width of 950 fs using a mode-locked semiconductor laser. ing. However, in general, a mode-locked semiconductor laser is often operated with a driving current that is slightly higher than a threshold for laser driving. Since such a semiconductor laser basically uses a laser oscillation operation, the carrier density at the time of oscillation is fixed at a threshold level. Therefore, the peak power of the optical pulse is about mW, for example, and it is difficult to increase its output.

  On the other hand, a saturable absorption region is provided in the resonator of the semiconductor laser, and the pulse width is 1.6 ps and the peak of the optical pulse is obtained by the passive Q switching method using the saturable absorption by applying the reverse bias voltage and the gain switching method. There is a report that an optical pulse having a power of 10 W was obtained (PPVasil'ev, IHWhite, D. Burns, and W. Sibbett, CLEO'93, Baltimore, Paper CThC8 (1993), Non-Patent Document 2). This method is characterized in that a peak power several tens of times higher than other mode synchronization methods can be obtained. Further, in the optical pulse generator described in Japanese Patent Laid-Open No. 10-229252 (Patent Document 1), the response speed of the saturable absorption operation is improved by providing a non-doped cladding layer in the vicinity of the active layer in the same configuration. Get a sub-picosecond light pulse.

  In a semiconductor laser using the passive Q switching method, for example, an n-electrode is deposited on the bottom surface of an n-type substrate, and an n-type buffer layer, n-type cladding layer, active layer, p-type cladding layer, and p-type cap layer are formed on the top surface. A laser is formed by sequentially stacking layers. A structure in which a p-type, n-type, or neutral active layer is sandwiched between p-type and n-type clad layers is called a double hetero (DH) structure.

  The p-type cap layer is provided with a ridge stripe-shaped protrusion along the longitudinal direction of the laser resonator, the protrusion is divided into three in the longitudinal direction, and two p-electrodes are formed on the p-type cap layer. Form. Then, one of the p-electrodes is stacked on the central protrusion of the three divided protrusions, and the other p-electrode is stacked on the protrusions at both ends. At this time, a different electric field can be applied between each of the two p electrodes and the n electrode by adopting a configuration in which the two p electrodes are not in contact with each other. As a result, the resonator portion of the semiconductor laser is electrically divided into three regions, and the center can be made a saturable absorption region and both sides can be gain regions.

That is, when a modulation current is applied as a laser drive current to the gain region among these regions, the optical pulse is modulated. Further, when a DC reverse bias voltage is applied to the saturable absorption region, the saturable absorption phenomenon is emphasized. Therefore, by using both of these, it is possible to realize a high output and short pulse of the optical pulse.
JP-A-10-229252 JP-A-1-150121 JP 2003-31897 A Optics 29, 8 p.495, 2000 PPVasil'ev, IHWhite, D. Burns, and W. Sibbett, CLEO'93, Baltimore, Paper CThC8 (1993) PPVasil'ev, H.Kan, H.Ohta, and T.Hiruma, "Experimental evidence of condensation of electron-hole pairs at room temperature during femtosecond cooperative emission", Phys. Rev. B Vol.64, p.195209-1 (2001)

  However, it cannot be said that a conventional semiconductor laser having a DH structure can sufficiently obtain a light pulse having a short pulse. That is, in the optical pulse generating element having the above structure using the passive Q switching method, there may be a timing jitter in which the light emission timing of the optical pulse is shifted for each pulse. When timing jitter occurs in this way, there is a problem that when integrated measurement is performed using an optical pulse, the pulse width is widened and observed.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an optical pulse generating element capable of stably obtaining a short pulse optical pulse.

  In order to achieve such an object, an optical pulse generating device according to the present invention has (1) a double heterostructure in which an active layer is laminated so as to be sandwiched between a first conductivity type cladding layer and a second conductivity type cladding layer. In addition, the laser beam resonator is configured in a direction orthogonal to the stacking direction of the double heterostructure, and (2) an electrode for supplying current to the active layer via the first conductivity type cladding layer in the longitudinal direction of the laser beam resonator Dividing the layer into at least two layers, applying a reverse bias voltage to at least one of the divided electrode layers to form a saturable absorption region, and applying a laser driving pulse current to at least one other electrode layer And (3) feedback means for feeding back a part of the optical pulse output from the laser optical resonator into the laser optical resonator with a predetermined delay time. And wherein the door.

  In the optical pulse generator described above, the response speed of the absorption region is improved by applying a reverse DC bias voltage to the saturable absorption region, and the rise is quick and short by applying an RF bias to the gain region. A pulsed light pulse can be obtained. Further, a part of the optical pulse output from the laser optical resonator is fed back with a predetermined delay time corresponding to the timing interval of the optical pulse train. As a result, a time correlation can be established between the output optical pulse and the next output optical pulse, thereby suppressing the occurrence of timing jitter and stabilizing the shortened optical pulse. Can be obtained. Moreover, such a feedback means can be comprised by the feedback optical system which has the optical path length corresponding to a required delay time, for example.

  Here, it is preferable that the feedback means feeds back the optical pulse output from one resonator end face of the laser optical resonator from the same resonator end face into the laser optical resonator. Thereby, feedback of the output optical pulse into the laser optical resonator can be realized with a simple configuration.

  Further, the feedback means may branch off a part of the optical pulse output forward from the laser optical resonator and feed it back into the laser optical resonator. Alternatively, the feedback means may feed back the optical pulse output backward from the laser optical resonator into the laser optical resonator. By using these configurations, the above feedback means can be suitably realized.

  According to the present invention, in an optical pulse generation device having a double hetero structure, a part of the output optical pulse is fed back into the laser optical resonator, so that a short pulse optical pulse can be stably obtained. Is possible.

  Hereinafter, preferred embodiments of an optical pulse generating device according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings do not necessarily match those described.

  First, the configuration of the semiconductor laser used in the optical pulse generating element according to the present invention will be described. FIG. 1 is a perspective view showing a configuration of an embodiment of a semiconductor laser used for an optical pulse generating element. 2 and 3 are a sectional view taken along the line II-II and a sectional view taken along the line III-III of the semiconductor laser shown in FIG. 1, respectively. Here, FIG. 2 is a cross-sectional view along the longitudinal direction of the laser resonator in the semiconductor laser.

First, the stacked structure of the semiconductor laser 10 according to the present embodiment will be described with reference to FIGS. The semiconductor laser 10 has a DH structure, and an n-type buffer layer 36 made of GaAs, an n-type cladding layer 37 made of Al _0.4 Ga _0.6 As, on the upper surface of an n-type substrate 35 made of GaAs, A non _- dope active layer 38 made of Al _x Ga _1-x As, a p-type cladding layer 40 made of Al _0.4 Ga _0.6 As, and a p-type cap layer 41 made of GaAs are sequentially stacked. An n electrode 44 is deposited on the lower surface of the n-type substrate 35. Here, x <0.4.

  Each layer on the n-type substrate 35 is laminated by, for example, the MO-CVD method. About the thickness of each layer, it is preferable to set the thickness of the active layer 38 in the range of 0.05-0.5 micrometer. The thicknesses of the cladding layers 37 and 40 are sufficient to confine light in the active layer 38, and are preferably about 2 μm. The n-type buffer layer 36 and the p-type cap layer 41 are each preferably about 1 μm thick.

  Further, as shown in FIG. 1, the p-type cap layer 41 is formed with a ridge stripe-shaped protrusion 30 along the longitudinal direction of the laser light resonator. The protrusion 30 is divided into three parts by removing the upper half of the p-type cap layer 41 at two locations in the longitudinal direction. Further, as shown in FIG. 3, an insulating SiN film 45 is deposited on the surface of the p-type cap layer 41 excluding the top surface portion of the protrusion 30. On the SiN film 45, two p-electrodes 42 and 43 each having a concave shape and a convex shape are stacked. These two p-electrodes 42 and 43 are arranged in such a manner that the concave portion and the convex portion face each other, and this portion is combined by the protruding portion 30. Therefore, of the three divided projections 30, a convex p-electrode 43 is laminated on the central projection 30, and a concave p-electrode 42 is laminated on the projections 30 at both ends. ing.

  These p electrodes 42 and 43 are in direct contact with the p-type cap layer 41 only at the top surface portion of the protrusion 30. In other portions, an insulating SiN film 45 is interposed between the p electrodes 42 and 43 and the p-type cap layer 41. For this reason, the region where current is injected into the active layer 38 is limited only to the portion directly under the stripe-shaped protrusion 30. As a result, the threshold current is reduced, and at the same time, the active layer 38 has a waveguide property for confining the laser light in the stripe region, thereby stabilizing the oscillating transverse mode. The waveguide length along the longitudinal direction of the laser resonator in this active layer 38 becomes the resonator length in this semiconductor laser. The waveguide end face (resonator end face) located in the longitudinal direction of the resonator is a cleavage plane in both directions.

  In the above structure, since the two p electrodes 42 and 43 are not in contact with each other, different electric fields can be applied between each of the p electrodes 42 and 43 and the n electrode 44. As a result, the semiconductor portion 34 is electrically divided into three regions, the center being the saturable absorption region 32 and the both sides being the gain region 31. The region excluding the saturable absorption region 32 and the gain region 31 is a separation region. That is, in the resonator portion having a cross-sectional structure shown in FIG. 2, the gain region 31 immediately below the p-electrode 42 on both sides is directly below the central p-electrode 43 via the isolation region 33 where the p-electrode is not formed on the surface. The saturable absorption region 32 is sandwiched.

  The gain region 31 includes all of the semiconductor portion 34, the p electrode 42, and the n electrode 44. The saturable absorption region 32 includes all of the semiconductor portion 34, a p-electrode 43, and an n-electrode 44. The isolation region 33 includes a portion left after the upper portion of the p-electrode and the p-type cap layer 41 is removed, another semiconductor portion, and an n-electrode 44.

  Here, in the semiconductor laser shown in FIG. 1, the waveguide length (resonator length) in the longitudinal direction of the laser light resonator is set in the range of 100 μm to 1500 μm, for example. Moreover, the length of the saturable absorption region 32 is set in a range of 10 μm or more and 500 μm or less, for example.

  Next, the operation of the semiconductor laser 10 shown in FIG. 1 will be described. FIG. 4 is a schematic diagram showing the operation of the semiconductor laser. Here, the semiconductor laser is illustrated by a cross-sectional view along the longitudinal direction of the laser light resonator similar to FIG.

  As schematically shown in FIG. 4, a current source for applying a pulse current to the gain region 31 is connected to the p electrode 42 corresponding to the gain region 31, and the p electrode 42 and the n electrode 44 are connected to each other. A laser driving pulse current is applied between them. A power supply for applying a voltage to the saturable absorption region 32 is connected to the p electrode 43 corresponding to the saturable absorption region 32, and a DC reverse bias voltage is connected between the p electrode 43 and the n electrode 44. Is applied.

  First, in the gain region 31 to which the laser driving pulse current is applied, electrons are gradually accumulated in the active layer 38 as the input current due to the pulse current increases. When the electron density exceeds the oscillation threshold, the photon density rises rapidly and laser oscillation occurs. As a result, the accumulated electron-hole pairs are rapidly consumed, and the photon density is rapidly decreased from the point when the oscillation threshold density falls below, thereby stopping the laser oscillation. As a result, as shown in FIG. 4, a laser light pulse having a pulse width shorter than the pulse width of the laser driving pulse current itself can be obtained.

  When the light pulse passes through the saturable absorption region 32, the light at the leading edge of the pulse is absorbed by the absorption region 32. However, when the light output exceeds a certain threshold value, the absorption rapidly decreases. The light pulse is transmitted through transparency. This is because the photoexcited electrons fill the conduction band of the absorption region 32 and there is no destination for transition. Thereafter, the photoexcited electrons in the absorption region 32 transit to another region or the like, the absorptance in the absorption region 32 rises again, and the light pulse is absorbed. As a result, the optical pulse is compressed.

  Here, when a bias voltage is not applied to the absorption region 32, it takes about 100 ns (nanoseconds), for example, to recover after light absorption is lost. On the other hand, when a reverse bias voltage is applied to the absorption region 32, minority carriers generated by light absorption can be ejected, and light absorption can be recovered at about 0.1 ps (picosecond) at the shortest. The reverse bias voltage is preferably -10V or more and -4V or less.

  As for the short pulse current applied to the gain region 31, in order to narrow the pulse width of the obtained optical pulse and increase the optical output, it is better that the pulse width is as short as 10 ns or less. Also, the larger the current value, the better. Specifically, the current value of the pulse current is preferably 0.4 A or more and 1 A or less, and the pulse width is 1 ns or more and 10 ns or less in consideration of the capability of the pulse current source used for laser driving.

  Further, in the optical pulse obtained by the semiconductor laser 10 having such a configuration, the light emission timing and the light emission timing interval (repetition frequency) are set by the timing and timing interval of the laser drive pulse current applied to the gain region 31. The That is, in the semiconductor laser 10, the optical pulse train is output at a timing interval corresponding to the timing interval of the short pulse current applied to the gain region 31. The timing interval of the optical pulse may be set arbitrarily, but can be set from several Hz to several hundred MHz, for example.

  Next, an optical pulse generating element according to the present invention using the semiconductor laser 10 described above will be described. FIG. 5 is a block diagram schematically showing the configuration of the first embodiment of the optical pulse generating element according to the present invention.

  This optical pulse generating element includes the semiconductor laser 10 having the configuration shown in FIG. 1, and a part of the optical pulse output from the laser optical resonator of the semiconductor laser 10 is transferred into the laser optical resonator with a predetermined delay time. Configured to provide feedback. In the present embodiment, as the output light fed back to the semiconductor laser 10, an optical pulse output forward from the laser optical resonator of the semiconductor laser 10 is used.

  Specifically, the optical pulse generating element shown in FIG. 5 includes feedback means including an optical branching unit 25 and a feedback optical system 21. The optical branching unit 25 is arranged at a predetermined position on the optical path between the cavity end face on the front side of the laser optical resonator in the semiconductor laser 10 and the measurement system irradiated with the optical pulse output from the semiconductor laser 10. A part of the optical pulse output from the semiconductor laser 10 forward (in the direction toward the measurement system) is branched and guided to the feedback optical system 21. Further, the optical pulse transmitted through the optical splitter 25 is guided to a measurement system that uses the optical pulse.

  The feedback optical system 21 has a delay having an optical path length set so as to obtain a delay time that coincides with a light pulse emission timing interval (a timing interval of a short pulse current applied to the gain region 31) in the semiconductor laser 10. It is an optical system. The optical pulse branched by the optical splitter 25 is given a predetermined delay time by the feedback optical system 21 and then fed back again from the front end face of the resonator into the laser optical resonator of the semiconductor laser 10.

  The effect of the optical pulse generating element according to the above embodiment will be described.

  In the optical pulse generating element shown in FIG. 5, the response speed of the absorption region 32 is improved by applying a DC bias voltage in the reverse direction to the saturable absorption region 32, and the RF bias is applied to the gain region 31. Thus, it is possible to obtain an optical pulse that is agile and has a short pulse. In addition, with respect to the light emission operation in the semiconductor laser 10, a part of the optical pulse output from the laser optical resonator is transmitted by the optical splitter 25 and the feedback optical system 21 with a predetermined delay time that matches the timing interval of the optical pulse train. It is configured to provide feedback. Thereby, a time correlation can be given between the outputted optical pulse and the next outputted optical pulse. Therefore, generation of timing jitter can be suppressed and a short pulse optical pulse can be stably obtained.

  That is, in the semiconductor laser 10 using the passive Q switching method having the configuration shown in FIG. 1, the light emission conditions such as the light emission timing of the optical pulse may be shifted from pulse to pulse and timing jitter may occur. Here, FIG. 6 is a graph showing the SHG autocorrelation waveform obtained by measuring the intensity autocorrelation of the optical pulse obtained by the semiconductor laser 10 shown in FIG. In this graph, the horizontal axis represents time (ps), and the vertical axis represents SHG intensity. Here, in the configuration shown in FIG. 1, the waveguide length in the longitudinal direction of the laser resonator is 350 μm, the stripe width of the protrusion 30 is 5 μm, the central saturable absorption region 32 is 80 μm in length, The region lengths of the gain regions 31 were each 125 μm.

  As shown in the graph of FIG. 6, in the optical pulse from the semiconductor laser 10, noise light is generated in the side lobe of the accumulated time waveform due to the influence of timing jitter. On the other hand, according to the optical pulse generating element according to the present invention that feeds back a part of the optical pulse via the feedback optical system 21, the laser oscillation in the gain region 31 in the operation of the semiconductor laser 10 described above with reference to FIG. As a result of the process of making the saturable absorption region 32 transparent, the optical pulse fed back affects the light emission operation in the laser optical resonator, and triggers the emission of the optical pulse. Therefore, the delay time due to the optical pulse feedback is synchronized with the timing interval of the optical pulse emission, and the next optical pulse is output with a time correlation with the previously output optical pulse. Can be controlled.

  FIG. 7 is a graph showing an SHG autocorrelation waveform by intensity autocorrelation measurement of an optical pulse obtained by performing feedback of the optical pulse with the optical pulse generating element shown in FIG. As shown in this graph, a part of the output optical pulse is fed back into the laser resonator of the semiconductor laser 10 at a timing that matches the light emission timing of the optical pulse, so that the timing jitter in the semiconductor laser 10 Is suppressed, and noise light is reduced. Further, the spread of the pulse width of the optical pulse due to timing jitter is also reduced. The delay time given to the optical pulse to be fed back depends on an integral multiple of the timing interval of the optical pulse train so that it is fed back into the laser optical resonator in accordance with the light emission timing of the optical pulse after two or more times. It may be a delay time.

  Further, for feedback of the optical pulse to the laser optical resonator of the semiconductor laser 10, the feedback means such as the feedback optical system 21 uses the optical pulse output from one resonator end surface of the laser optical resonator as the same resonator end surface. It is preferable to feed back into the laser optical resonator by inputting again from the above. Thereby, feedback of the output optical pulse into the laser optical resonator can be realized with a simple configuration.

  Specifically, the optical pulse generating element shown in FIG. 5 can be realized by various configurations. FIG. 8 is a block diagram showing a specific example of the optical pulse generating element shown in FIG. In the present configuration example, the feedback optical system 21 includes four reflection mirrors 21a to 21d.

  The optical pulse output from the resonator end face 10a on the front side of the semiconductor laser 10 and branched by the partially reflecting mirror 25, which is an optical splitter, is separated by a predetermined distance (optical path length) via the reflecting mirror 21a. Then, the light is incident on a delay optical system including a pair of reflecting mirrors 21b and 21c. Then, the light pulse that has been multiple-reflected between the reflection mirrors 21b and 21c a predetermined number of times is reflected by the reflection mirror 21d toward the original optical path, and again, the reflection mirrors 21b and 21c, the reflection mirror 21a, and the partial reflection mirror As a feedback light pulse to which a predetermined delay time is given, the feedback is provided from the resonator end face 10 a to the laser light resonator of the semiconductor laser 10.

  Here, the short pulse light emission in the semiconductor laser 10 shown in FIG. 1 may be caused by the light emission mechanism (solid arrow) shown in FIG. 9 depending on the setting of the element structure and operating conditions. This is because when electron-hole pairs are present at a very high density (corresponding to several times the laser carrier density), a level degenerated to the lowest energy level in the normal GaAs band is formed. Coherent recombination. Such a light emission mechanism is said to be super-radiation. For example, PPVasil'ev, H.Kan, H.Ohta, and T.Hiruma, Phys. Rev. B Vol.64, p.195209-1 (2001) (non-patent The physics has been clarified in the literature 3).

  In such a light emitting mechanism, the shortening of the pulse, which is limited by the carrier lifetime in normal laser oscillation, breaks the limit, and a pulse width of, for example, several hundred fs can be obtained. On the other hand, when this super-radiation is used as a light emission mechanism in the semiconductor laser 10, such a light emission mechanism tends to have a large fluctuation in light emission operation and a large timing jitter due to its principle. Specifically, considering an ideal inversion distribution with a high density formed in a very short time, the emission timing of an optical pulse due to coherent recombination in a superradiance process is affected by quantum fluctuations.

  On the other hand, according to the optical pulse generating element provided with the self-delay feedback optical system 21 shown in FIG. 5, the timing at which the optical pulse is fed back is synchronized with the emission timing of the next optical pulse, The light emission operation of the semiconductor laser 10 can be controlled so as to have a time correlation. As a result, even when the influence of quantum fluctuations in the superradiation process becomes a problem, the timing jitter for each optical pulse can be controlled to be constant.

  The light emission mechanism by super radiation will be briefly described. The generation of super-radiant light is described, for example, by Shinichi Hanamura, “Quantum Optics”, Chapter 5-2 (Iwanami Shoten), Shuurmns et al., “Superfluorescence”, Adv. Atom. Molecul. Phys. 17, p.176 (1981) As described in the above, it has been observed using HF molecular gas or Cs atom beam as a medium. For example, by irradiating a 100 ns laser light pulse on a cell in which HF molecular gas is sealed under pressure control, the HF molecules are excited to form an inversion distribution. At this time, since light emission from each excited molecule is independent when the pressure in the cell is low, pulse light emission of about 1 s determined by the light emission lifetime of the molecule is observed. This is so-called incoherent light emission.

On the other hand, when the pressure in the cell is increased, a sharp pulse emission with a delay time of about 1 μs and a width of 200 ns is obtained, and the intensity reaches ¹⁰ × ¹⁰ times. The emission peak intensity at this time is proportional to the square of the pressure of the HF molecular gas. From these experimental facts, it is understood that the light emission having this sharp peak is not spontaneous emission light, spontaneous emission amplification (ASE) light, or laser oscillation light, but superradiation light as coherent spontaneous emission of excited molecules. ing.

  Super-radiant light is characterized by beam emission with directivity, sharp peak emission with a delay time, pulse emission with a very large peak intensity compared to normal natural emission, and the intensity of the number of particles involved in the emission. It is proportional to the square. Further, the peak intensity, the pulse shape, and the delay time change depending on the excitation intensity.

The super-radiation occurs under the condition that the inversion distribution is formed in a shorter time with respect to the super-radiation delay time τ _D , the time τ _E = L / c when the light escapes from the excitation field is the longitudinal relaxation time T 1 of the excited particle, It must be sufficiently shorter than the lateral relaxation time T2, and the pulse width τ _R and the delay time τ _D must satisfy τ _E <τ _R <τ _D <T1, T2. For example, in the experiment by Gibbs et al. By laser excitation of Cs atoms, the conditions are τ _E = 0.067 <τ _R = 5 <τ _D = 10 <T1 = 70 and T2 = 80 (ns).

  As described above, the relaxation time of HF gas and Cs atoms is on the order of nanoseconds. Also, super-radiation cannot be realized unless the environment is special, such as excitation with a special gas laser, control of gas pressure, and handling of an atomic beam. There are also questions regarding its stability and controllability. On the other hand, in the above-described optical pulse generating element, it is possible to realize generation of superradiant light in a new electron-hole state in a semiconductor.

  Patent Documents 2 and 3 describe a configuration in which an optical pulse is input from the outside to a semiconductor laser provided with a gain region and a saturable absorption region. However, in Patent Document 2, the semiconductor laser is used as an optical limiter that inputs an optical pulse from an optical bandpass filter including an optical loop circuit and generates an optical clock signal. Moreover, in patent document 3, the light pulse from the light source provided separately is input with respect to the semiconductor laser. That is, in the configurations described in these Patent Documents 2 and 3, the light emission operation of the semiconductor laser is controlled by inputting the light pulse generated by the external device.

  On the other hand, in the optical pulse generating device according to the present invention, the semiconductor laser 10 is not input with an optical pulse from an external device to the semiconductor laser 10 provided with the gain region 31 and the saturable absorption region 32. A part of the light pulse output from the laser light resonator is fed back. According to such a configuration, in the optical pulse train generated by the semiconductor laser 10, the optical pulse train in which the occurrence of timing jitter is sufficiently suppressed can be realized with a simple configuration by providing time correlation between different optical pulses. It becomes possible.

  FIG. 10 is a block diagram schematically showing the configuration of the second embodiment of the optical pulse generating element according to the present invention.

  This optical pulse generating element includes the semiconductor laser 10 having the configuration shown in FIG. 1, and a part of the optical pulse output from the laser optical resonator of the semiconductor laser 10 is transferred into the laser optical resonator with a predetermined delay time. Configured to provide feedback. In the present embodiment, as the output light fed back to the semiconductor laser 10, an optical pulse output backward from the laser light resonator of the semiconductor laser 10 is used. In other words, the semiconductor laser 10 outputs constant light not only in the front direction but also in the rear direction (the direction opposite to the measurement system). In this optical pulse generating element, it is used as an optical pulse for feeding back the backward light.

  Specifically, the optical pulse generating element shown in FIG. 10 is provided with feedback means including a feedback optical system 22. The feedback optical system 22 is a delay optical system having an optical path length set so as to obtain a delay time that coincides with the light pulse emission timing interval in the semiconductor laser 10. The optical pulse output from the rear end face of the laser light resonator in the semiconductor laser 10 is given a predetermined delay time by the feedback optical system 22, and then again from the rear end face of the resonator 10. Is fed back into the laser beam resonator.

  The effect of the optical pulse generating element according to the above embodiment will be described.

  In the optical pulse generating element shown in FIG. 10, the optical pulse output as the backward light from the laser resonator is matched with the timing interval of the optical pulse train by the feedback optical system 22 for the light emission operation in the semiconductor laser 10. The feedback is performed with a predetermined delay time. As a result, in the same manner as the configuration of FIG. 5 in which a part of the optical pulse output forward is fed back by the optical branching unit 25 and the feedback optical system 21, the output optical pulse and the optical pulse output next are output. A time correlation can be given between Therefore, generation of timing jitter can be suppressed and a short pulse optical pulse can be stably obtained.

  In such a configuration, it is not necessary to provide the optical branching unit 25 as shown in FIG. 5 for the optical pulse supplied from the semiconductor laser 10 to the measurement system. Thereby, the output optical pulse can be efficiently supplied to the measurement system. In addition, the configuration of the optical pulse generating element can be simplified. The intensity of the optical pulse used for feedback can be set by the reflectivity at the resonator end face on the rear side of the semiconductor laser 10 or the like.

  FIG. 11 is a block diagram showing a specific example of the optical pulse generating element shown in FIG. In the present configuration example, the feedback optical system 22 is configured by a feedback optical fiber 22a. The optical fiber 22 a is optically connected to the resonator end face 10 b on the rear side of the semiconductor laser 10.

  The optical pulse output from the resonator end face 10b on the rear side of the semiconductor laser 10 is incident on the delay optical system including the optical fiber 22a. A reflection coating 22b is applied to the end of the optical fiber 22a opposite to the semiconductor laser 10. The light pulse that has passed through the optical fiber 22a is reflected by the reflective coating 22b toward the original optical path, and is again transmitted from the resonator end face 10b through the optical fiber 22a as a feedback light pulse to which a predetermined delay time is given. This is fed back into 10 laser resonators. For example, if a single mode fiber having a length of 5 to 10 m is used as the optical fiber 22a, a delay time of about 20 to 30 ns can be provided.

  The optical pulse generating element according to the present invention is not limited to the above-described embodiments and examples, and various modifications can be made. For example, regarding the specific configuration of the feedback optical system that feeds back an optical pulse, various configurations other than the embodiments shown in FIGS. 8 and 11 may be used. Alternatively, a configuration may be adopted in which an optical pulse output from one resonator end face of the semiconductor laser is input from the other resonator end face.

  In addition to the configuration shown in FIG. 1, various configurations can be used as the configuration of the semiconductor laser used for the optical pulse generating element.

For example, with respect to the laminated structure of the semiconductor laser, as shown in FIG. 12 with a cross-sectional view similar to FIG. 2 as a modification of the semiconductor laser 10 shown in FIG. A non-doped clad layer 39 (where x <y <0.4) made of Al _y Ga _1-y As may be provided therebetween. By providing the non-doped cladding layer 39 in this way, the thickness of the depletion layer formed by application of the reverse bias voltage to the saturable absorption region 32 becomes thicker than in the case of only the active layer 38, so that the absorption region The response speed of 32 is increased, and the obtained optical pulse width can be shortened.

  The semiconductor laser 10 shown in FIG. 1 uses a structure in which the saturable absorption region 32 is sandwiched between two gain regions 31 in the configuration along the longitudinal direction of the laser light resonator. Such a configuration is characterized in that the laser light pulse collides front in the absorption region 32 and the saturation effect is enhanced. However, such a configuration may be another configuration.

  FIG. 13 is a perspective view showing the configuration of another embodiment of the semiconductor laser used in the optical pulse generating element according to the present invention. Here, also in the semiconductor laser 11 shown in FIG. 13, the laminated structure is the same as that shown in FIG.

  In the present embodiment, as shown in FIG. 13, the p-type cap layer 41 is formed with a ridge stripe-shaped protrusion 50 along the longitudinal direction of the laser light resonator. The protruding portion 50 is divided into two parts by removing the upper half of the p-type cap layer 41 at one central portion in the longitudinal direction. An insulating SiN film 45 is deposited on the surface of the p-type cap layer 41 except for the top surface portion of the protrusion 50. On the SiN film 45, two p-electrodes 46 and 47 each having a convex portion on the right side toward the protrusion 50 are stacked. These two p-electrodes 46 and 47 are arranged in such a manner that the respective convex portions are combined at the protrusion 50. Therefore, of the two divided protrusions 50, the p-electrode 47 is stacked on the front protrusion 50 and the p-electrode 46 is stacked on the rear protrusion 50 in FIG.

  These p-electrodes 46 and 47 are in direct contact with the p-type cap layer 41 only at the top surface portion of the protrusion 50. In other portions, an insulating SiN film 45 is interposed between the p electrodes 46 and 47 and the p-type cap layer 41. Further, since the two p electrodes 46 and 47 are not in contact with each other, different electric fields can be applied between the p electrodes 46 and 47 and the n electrode 44. As a result, the semiconductor portion 34 is electrically divided into two regions, and the front is a saturable absorption region 52 and the rear is a gain region 51. The region excluding the saturable absorption region 52 and the gain region 51 is a separation region. That is, in the resonator portion, the gain region 51 immediately below the rear p-electrode 46 is opposed to the saturable absorption region 52 immediately below the front p-electrode 47 through the separation region where the p-electrode is not formed on the surface. It has a structure to do. The operation of the semiconductor laser 11 is the same as that of the embodiment shown in FIG. As described above, various structures can be used for the arrangement structure of the gain region and the saturable absorption region.

  In any of the semiconductor lasers shown in FIG. 1 and FIG. 13, an HR / AR coating may be applied to the waveguide end face of the resonator formed of a cleavage plane as necessary. However, it is preferable to design so as to be advantageous for shortening the pulse and increasing the output in consideration of the interference between the forward wave and the backward wave.

  INDUSTRIAL APPLICABILITY The present invention can be used as an optical pulse generating element that can suppress the generation of timing jitter and can stably obtain a shortened optical pulse.

It is a perspective view which shows the structure of one Embodiment of the semiconductor laser used for an optical pulse generation element. It is II-II arrow sectional drawing of the semiconductor laser shown in FIG. FIG. 3 is a cross-sectional view of the semiconductor laser shown in FIG. 1 along arrows III-III. It is a schematic diagram shown about operation | movement of the semiconductor laser shown in FIG. It is a block diagram which shows the structure of 1st Embodiment of an optical pulse generation element. It is a graph which shows the SHG autocorrelation waveform by intensity | strength autocorrelation measurement of the optical pulse obtained with the semiconductor laser shown in FIG. 6 is a graph showing an SHG autocorrelation waveform obtained by measuring the intensity autocorrelation of an optical pulse obtained by feeding back an optical pulse with the optical pulse generating element shown in FIG. 5. It is a block diagram which shows the specific Example of the optical pulse generation element shown in FIG. It is a figure which shows the light emission mechanism in an optical pulse generation element. It is a block diagram which shows the structure of 2nd Embodiment of an optical pulse generation element. It is a block diagram which shows the specific Example of the optical pulse generation element shown in FIG. It is sectional drawing which shows the modification of the semiconductor laser shown in FIG. It is a perspective view which shows the structure of other embodiment of the semiconductor laser used for an optical pulse generation element.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10, 11 ... Semiconductor laser, 21, 22 ... Feedback optical system, 25 ... Optical splitter, 30 ... Projection part, 31 ... Gain area | region, 32 ... Saturable absorption area | region, 33 ... Separation area | region, 34 ... Semiconductor part, 35 ... n-type substrate, 36 ... n-type buffer layer, 37 ... n-type cladding layer, 38 ... non-doped active layer, 39 ... non-doped cladding layer, 40 ... p-type cladding layer, 41 ... p-type cap layer, 42, 43 ... p electrode 44 ... n electrode, 45 ... SiN film, 46, 47 ... p electrode, 50 ... projection, 51 ... gain region, 52 ... saturable absorption region.

Claims (4)

A laser beam resonator having a double hetero structure in which an active layer is laminated so as to be sandwiched between a first conductivity type clad layer and a second conductivity type clad layer;
An electrode layer that supplies current to the active layer via the first conductivity type cladding layer is divided into at least two with respect to the longitudinal direction of the laser resonator, and at least one of the divided electrode layers. A reverse bias voltage is applied to one of the electrode layers to form a saturable absorption region, a laser driving pulse current is applied to at least one other electrode layer to form a gain region, and
An optical pulse generating element comprising feedback means for feeding back a part of an optical pulse output from the laser optical resonator into the laser optical resonator with a predetermined delay time.   The feedback means feeds back an optical pulse output from one resonator end face of the laser optical resonator into the laser optical resonator by inputting again from the same resonator end face. The optical pulse generating element according to 1.   3. The optical pulse according to claim 1, wherein the feedback means branches a part of the optical pulse outputted forward from the laser optical resonator and feeds it back into the laser optical resonator. Generating element.   3. The optical pulse generating element according to claim 1, wherein the feedback means feeds back an optical pulse output backward from the laser optical resonator into the laser optical resonator.

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