Optical bistable laser diode and a controlling method thereof

Abstract: An optical bistable laser diode comprises a first semiconductor clad layer (22) of a first conduction type, a second semiconductor clad layer (24) of a second conduction type, an active semiconductor layer (22) of intrinsic type provided so as to be sandwiched between the first and second semiconductor clad layers, reflectors (E₁, E₂) provided on first and second ends of the active semiconductor layer, first and second electrodes (28a, 28b) provided on the second semiconductor clad layer substantially in alignment in a direction connecting the first end and the second end, and a gap region (L₃) defined in the second semiconductor clad layer between the first and second electrodes, the gap region defining a saturable absorption region (31) in a part of the active semiconductor layer located underneath, wherein the first electrode, second electrode and the gap region respectively have lengths L1, L2 and L3 satisfying a relation (L2 + L3/2)/(L1 + L2 + L3 ≦ 0.3.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to optical bistable laser diodes and more particularly to a structure of such a laser diode and a control method thereof for operating it at a high speed. Recently, the optical bistable laser diode receives attention as an essential element for constructing optical logic devices or optical memory devices for use in optical information processing systems or optical computers. Such a device having optical bistability is suited for optical information processing based on the binary logic as the state of the device is switched between an optical high level state and an optical low level state similarly to a flip-flop used in the conventional electronic processing system. FIG.1 shows a typical prior art bistable laser diode. Referring to the drawing, the device comprises a clad layer 11 of a semiconductor material doped to a first conduction type, an active layer 12 of undoped semiconductor material provided on the clad layer 11, and another clad layer 13 doped to another conduction type further provided on the active layer 12. On the clad layer 13, there are provided first and second electrodes 14a and 14b forming a tandem electrode structure respectively in correspondence to a gain region 15 and a control region 17 both defined in the active layer 12. The gain region 15 is supplied with a drive current I₁ through the first electrode 14a and produces optical radiation by photon emission. The control region 17 is also supplied with a control current I₂ through the second electrode 14b and controls the overall gain of the laser diode. In the prior art device, the level of the control current I₂ is set such that the control region 17 operates as a second gain region. In other words, the control region 17, too, produces optical radiation with a controlled gain by emitting photon. Further, there is defined a saturable absorption region 16 between the gain region 15 and the control region 17. This region 16 changes its transmittance responsive to the optical radiation in the active layer 12 such that the transmittance of the layer 12 is increased responsive to increase of the optical radiation in the active layer 12 and such that the transmittance is decreased responsive to the decrease of the optical radiation in the active layer 12. Furthermore, there is provided a rear side electrode 18 at the bottom of the clad layer 11 for collecting electrical current injected through the first and second electrodes 14a and 14b. Note that there are also formed a pair of mirrors E₁ and E₂ at both ends of the laser diode to form a Fabry-Pérot resonator as usual. In a typical example, the laser diode has an overall length of 300 µm across the mirrors and the gain region 15 has a length of 192 µm, the saturable absorption region 16 has a length of 38 µm and the control region 17 has a length of 70 µm. Next, the operation of this prior art optical bistable laser diode and its problem will be described. In operation, the gain region 15 and the control region 17 are supplied at first with the drive current I₁ and the control current I₂ respectively as already described such that there is a photon emission in both of the regions 15 and 17. When the overall gain of the regions 15 and 17 exceeds the loss in the saturable absorption region 16 as well as the loss at the mirrors E₁ and E₂ the laser starts to oscillate while when the loss exceeds the gain in the regions 15 and 17, the oscillation of the laser stops. More specifically, in a first optical bistable state corresponding to the optical low level state of the laser diode, the optical radiation established in the active layer 12 as a result of the photon emission in the regions 15 and 17 is insufficient to cause transition of the saturable absorption region from opaque to transparent and there occurs no laser oscillation. As a result, there is obtained only a small optical output due to the electroluminescence. In a second bistable optical state corresponding to the optical high level state, the saturable absorption region becomes substantially transparent as a result of the strong optical radiation established in the active layer 12 and a large optical output power is obtained associated with the decrease of the loss in the saturable absorption region. FIG.2 is a characteristic curve of the laser diode of FIG.1 showing the optical output as a function of an overall current I injected to the laser diode. This current I is a sum of the drive current I₁ and the control current I₂ and is first set at a current level I B as shown in the drawing. The laser diode is turned on by increasing the overall current I beyond a turn-on level I ON and is turned off by decreasing the overall current I below a turn-off level I OFF . Such a turn-on and turn-off of the laser diode can be achieved by increasing the control current I₂ so that the overall current I exceeds the turn-on level I ON and by decreasing the control current I₂ so that the overall current I falls below the turn-off level I OFF . In other words, the laser diode is turned on by supplying a set current pulse having a level I ON - I B or more to the control region 17 in addition to the foregoing current I₂ and is turned off by supplying a reset current pulse having a level I B - I OFF to the control region 17. As the level I ON and I OFF are not identical, there appears a hysteresis which characterizes the bistable operation of the laser diode. Thus, in the turned-on state, the optical output power of the laser diode assumes a level O corresponding to the optical high level state while in the turned-off state, the optical output power assumes a level P corresponding to the optical low level state. Alternatively, the laser diode of FIG.1 may be turned on by injecting an external optical beam P in having a wavelength chosen so as to cause interaction with the material forming the active layer 12. More specifically, the optical beam P in has a wavelength or an energy which is equal to or larger than the band gap in the active layer 12. In this case, the current I₁ and I₂ are set such that the overall current I assumes the level I B similarly to the foregoing case and the laser diode is triggered by injecting the optical beam. When the optical beam called a set optical pulse is injected, the saturable absorption region 16 absorbs the optical beam and generates carrier. The carrier thus generated is accumulated in the region 16. As a result, the saturable absorption region 16 reduces its absorption coefficient and the region 16 changes from opaque to transparent. Thus, there is established a strong laser oscillation and an optical output corresponding to the optical high level state is established. Further, by applying the reset current pulse already described to the control region 17, the carrier is removed from the region 17 and the oscillation of the laser diode is stopped because of the loss in the control region 17 which is now depleted with the carrier. Note that the saturable absorption region 16 recovers its original transmittance or opacity responsive to the decrease of the optical power in the active layer 12. In such a prior art optical bistable laser diode, it is necessary to remove the carrier from the control region 17 as quickly as possible in order to obtain a quick turn-off. For this purpose, a very large negative reset current pulse has been used. For example, the current I₂ injected to the control region 17 is almost shut down in correspondence to the reset pulse. However, such a large reset pulse causes a strong depletion of carrier in the control region 17 and there is needed an excessively large optical power for the set optical pulse to turn on the laser diode particularly when the laser diode is to be turned on immediately after turn-off. Otherwise, one has to wait for a long time until the carrier density in the active layer 12 returns to a stationary state. This means that there appears a dead time responsive to the reset current pulse as illustrated in FIG.3 as long as an ordinary or small optical power is used for the optical set pulse. Generally, this dead time has a duration in the order of about several nanoseconds and prohibits the high speed operation of the device. In the optical computers and other information processing systems, it is necessary to operate the optical bistable device with a speed as high as possible without using excessive optical power for the set optical pulse, as the use of such a large optical power increases the size and power consumption of the system. From this view point, the prior art optical bistable laser diode is inappropriate for the optical computers. An optical bistable device which can be turned on and turned off at a high speed without using large optical power for the set optical beam is strongly demanded. Other examples of optical bistable devices corresponding substantially to the preamble of claim 1 can be found in JP-A-63 156 385 and in IEEE Proceedings, Section A, vol. 133, No. 4, pt. J, August 1986, pp. 283-292, Stevenage, Herts, GB; M.C. Perkins et al. : "Analysis of absorptive bistable characteristics of multisegment lasers". SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide a novel and useful optical bistable laser diode and a control method thereof wherein the aforementioned problems are eliminated. Another and more specific object of the present invention is to provide an optical bistable laser diode having an improved operational speed and a control method thereof for obtaining a high operational speed. Another object of the present invention is to provide an optical bistable laser diode and a control method thereof in which a dead time appearing responsive to resetting by a reset electrical pulse is substantially reduced without increasing the power of optical pulse used to turn on the laser diode. Another object of the present invention is to provide a method of controlling an optical bistable laser diode comprising an active layer sandwiched by a pair of semiconductor clad layers and a tandem electrode structure comprising a first electrode provided on one of the clad layers in correspondence to a gain region defined in the active layer for producing optical radiation with a predetermined optical gain and a second electrode provided on said clad layer in correspondence to a control region defined in the active layer for controlling an overall optical gain of the laser diode, with a saturable absorption region formed in the active layer between said gain region and said control region so as to change its optical absorption coefficient responsive to the optical radiation existing in the active layer, wherein said method comprises steps of injecting a predetermined drive current to the gain region via said first electrode for establishing said predetermined optical gain therein, applying a predetermined finite voltage via said second electrode to the control region with a voltage level determined such that there is no substantial current flowing through the control region and such that there exists an optical loss in said control region, switching the laser diode to a turn-on state by injecting an optical pulse to the active layer, and switching the laser diode to a turn-off state by applying a voltage lower than said predetermined finite voltage to the control region via the second electrode in a form of reset pulse. According to the present invention, the control region is always operated as a loss region by applying the predetermined voltage determined such that there is no virtual electrical current flowing through the control region. In other words, even when there is a generation of carrier in the control region as a result of the optical radiation supplied thereto from the gain region under the turned-on state of the laser diode, the generated carrier is immediately dissipated as a current and the loss in the control region is maintained. As a result, the magnitude of change of the carrier density taking place in the control region responsive to the turn-off of the laser diode is reduced to less than about one-tenth of that of the prior art device and the turn-on of the laser diode succeeding to the turn-on is achieved quickly by using a low power optical pulse. Associated therewith, the dead time appearing responsive to the reset pulse is reduced without increasing power consumption and the laser diode can be turned on and turned off repeatedly at a substantially high speed. Another object of the present invention is to provide an optical system comprising an optical bistable laser diode, as defined in claim 5. The invention also relates to methods of controlling optical bistable laser diodes as defined in claims 1 and 9. Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a cross sectional view showing structure of a prior art optical bistable laser diode; FIG.2 is a graph showing a characteristic curve with hysteresis pertinent to the laser diode of FIG.1; FIG.3 is a waveform chart showing a dead time pertinent to the laser diode of FIG.1; FIG.4 is a perspective view showing an embodiment of the optical bistable laser diode of the present invention; FIG.5 is a characteristic curve of a p-n junction used in the laser diode of FIG.4; FIG.6 is a diagram showing change of gain coefficient and absorption coefficient responsive to the change of carrier density in a semiconductor material used in the devices of FIGS.1 and 4; FIGS.7(A) and (B) are diagrams showing a relation between optical output power and injected current and a relation between optical output power and applied voltage for the laser diode of FIG.4; FIG.8 is a waveform chart showing an optical response of the laser diode of FIG.4 responsive to a reset voltage pulse and an optical set pulse; FIGS.9(A) and (B) are waveform charts showing a reduced dead time for the present invention and a prolonged dead time for the prior art device; FIG.10 is a graph showing reduction of the recovery time achieved in the present invention in comparison with the prior art device; and FIGS.11(A) - (D) are diagrams showing steps of manufacturing the laser diode of FIG.4. DETAILED DESCRIPTION FIG.4 shows an embodiment of the optical bistable laser diode of the present invention. Referring to the drawing, the laser diode comprises a substrate 21 of indium phosphide (InP) doped to the n-type. The substrate 21 is defined by a front end E₁ and a rear end E₂ and there is provided a clad layer 22 of InP also doped to the n-type on the substrate 21. The clad layer 22 is formed with a central part 22′ extending centrally through the the clad layer 22 from the front end E₁ to the rear end E₂ with a limited lateral width W and an active layer 23 of undoped indium gallium arsenide phosphide (InGaAsP) is provided on the central part 22′ with a width identical to the width W from the front end E₁ to the rear end E₂. On the active layer 23, there is provided another clad layer 24 of InP doped to the p-type so as to extend from the front end E₁ to the rear end E₂ with a lateral width identical to the width W . Further, a contact layer 25 of InGaAsP doped to the p-type is provided on the clad layer 24 so as to extend along the clad layer 24 with a lateral width identical to the width W . In the illustrated example, the width W is chosen to 1.5 µm. Further, a structural part 37 comprising the central part 22′ of the clad layer 22, the active layer 23, the clad layer 24 and the contact layer 25 is laterally supported or bounded by a pair of semi-insulating buried layers 26 of indium phosphide. On the structure thus described, there is further provided an insulator film 27 of silicon oxide (SiO₂) except for a central part or opening 34 extending along the foregoing structural part 37. In other words, the contact layer 25 is exposed along the central opening 34. Furthermore, a first electrode 28a is provided on a first region of the buried layers 26 close to the first end E₁ for making contact with the exposed contact layer 25 and a second electrode 28b is provided on a second region of the buried layers 25 close to the rear end E₂ for making contact with the exposed contact layer 25. Also, a back-side electrode 29 is provided on the bottom of the substrate 21. The electrodes 28a and 28b are formed so as to make an ohmic contact with the p-type contact layer 25 and comprise stacking of a first layer 35a of titanium and platinum and a second layer 35b of gold. The electrode 29 on the other hand comprises a first layer 36a which in turn comprises a stacking of gold-germanium alloy and gold and a second layer 36b of gold and makes the ohmic contact with the n-type substrate 21. Further, a pair of mirrors are formed at the both ends E₁ and E₂ of the laser diode as usual. The mirrors may be formed by polishing or simply cleaving the ends E₁, E₂. In order to avoid short circuit conduction across the electrode 28a and 28b through the contact layer 25, a part of the contact layer 25 extending in correspondence to the saturable absorption region 31 is removed as illustrated by a reference numeral 25b. In the laser diode of FIG.4, the first and second electrodes 28a and 28b form a tandem electrode structure and in correspondence to the first electrode 28a, there is formed a gain region 30 in the active layer 23 for producing optical radiation with a predetermined optical gain by emitting photon into the active layer. For this purpose, a constant current source 44 is connected to the electrode 28a. Further, there is formed a control region 32 in the active layer 23 in correspondence to the second electrode 28b for controlling an overall gain of the laser diode by absorbing the photon. In the gain region 30, a drive current I is supplied from the constant current source 44 via the first electrode 28a with a level sufficient to cause the laser oscillation. In the control region 32, on the other hand, a finite voltage V , set to a level such that there is no substantial current flowing through the region 32, is supplied through the second electrode 28b. As there is no substantial electrical current flowing through the control region 32, there is no photon emission in the control region 32 and the region 32 acts as a loss region. In order to set the control region 32 as such, the laser diode of the present invention uses a drive circuit 40 which operates as a constant voltage source producing the foregoing control voltage V . This constant voltage source 40 is connected to the second electrode 28b via a choke coil 40a. Further, a resistor 41 having a small resistance is connected for impedance matching. Because of the drive circuit 40 and the resistor 41, the carrier formed in the control region 32 as a result of absorption of the optical radiation is immediately dissipated to the ground and the control region 32 always acts as the loss region. Between the first and second electrodes 28a and 28b, there is formed a saturable absorption region 31 which changes the absorption coefficient or transmittance responsive to the optical radiation existing in the active layer 23 similarly to the saturable absorption region 16 of the prior art device. Thus, the saturable absorption region 31 becomes transparent when there is a sufficient optical power in the active layer 23. When the set optical pulse is injected to the active layer 23, therefore, there appears a transition from opaque to transparent in the saturable absorption region 31 and as a result, the state of the laser diode of FIG.4 is switched bistably to a high optical level state. In the optical bistable laser diode of the present invention, it is preferred that the control region 32 acting as the loss region is set as small as possible so as to obtain efficient laser oscillation. If the control region 32 is too large, not only that the turn-on of the laser diode becomes difficult but also the optical bistability due to the saturable absorption region 32 is lost because of the excessive loss in the control region 31. In the present invention it was found that the laser diode performs the optical bistable operation effectively when the following relation is satisfied: (L2 + L3/2)/(L1 + L2 + L3) ≦ 0.3, where L1 stands for the length of the gain region 30, L2 stands for the length of the control region 32 and L3 stands for the length of the saturable absorption region 32. In one preferred example, the length L1 was set to 256 µm, the length L2 was set to 16 µm and the length L3 was set to 28 µm. In this case, there holds a relation (L2 + L3/2)/L ≅ 0.1 where L = L1 + L2 + L3. By taking the length L3 as such, there is secured a resistance of about 8 kΩ between the first and second electrodes 28a and 28b and the interference between the gain region 30 and the control region 32 is avoided. Note that the buried layer 26 of indium phosphide which connects the first electrode 28a and the second electrode 28b electrically has a large resistivity ( > 10⁸ Ωcm). Next, the principle of operation of the optical bistable laser diode of the present invention will be described. FIG.5 is a voltage versus current characteristic of the control region 32. As the laser diode has a p-n junction therein, the voltage versus current characteristic has a feature pertinent to the p-n junction of diode in which the current increases steeply when the voltage V applied to the second electrode 28b has exceeded a predetermined level. Such a point of steep current rise is represented in FIG.5 by a point A and will be referred to as "knee point". The voltage at the knee point A is related to the diffusion potential of the p-n junction which generally takes a value of about 0.8 volts. The current flowing through the diode at the knee point A is, in the case of the material used for the control region 32, less than 100 µA. In correspondence to the knee point, a voltage of about 0.55 volts is applied to the second electrode 28b. The current corresponding to this voltage is about 10 µA. Under such a condition, there is substantially no photon emission in the control region 32 and the region acts as the loss region causing absorption of optical radiation passing therethrough. As already noted, the carrier formed in the control region 32 as a result of absorption of the optical radiation is immediately dissipated through the resistor 41 as well as through the voltage source 40 and the loss in the control region is maintained. In one example, the resistor 41 has a resistance of 50Ω. FIG.5 also shows the operational point of the prior art laser diode by a point B . Note that the current I₂ injected to the control region 17 of the device of FIG.1 is much larger than the current injected into the control region 32 of the device of FIG.4. Associated therewith, the prior art device needs a large reset current pulse to reset the laser diode as shown by a loop C . In operation, a drive current I having a level sufficient to cause oscillation of the laser diode is supplied to the gain region 30 through the first electrode 28a. As will be described later, the level of the current I is set at a center of hysteresis curve characterizing the optical bistability. In one example, a current of about 45.5 mA is supplied to the gain region 30 as the current I . Further, a voltage V corresponding to a half of the voltage of the knee point A on the voltage versus current characteristic for the control region 32 is applied to the second electrode 28b. Because of the loss in the control region 32, the laser diode does not start oscillation spontaneously even when there is supplied the current I with the level capable of causing laser oscillation. When it is desired to turn the laser diode on, an optical pulse called a set optical pulse is injected to the active layer 23 from outside. This set optical pulse has a wavelength close to or equal to the oscillation wavelength of the laser diode and thus interacts with the material forming the active layer 23. Responsive thereto, there appear carrier comprising holes and electrons in the saturable absorption region 31 as well as in the control region 32. The carrier formed in the control region 32 is immediately dissipated through the second electrode 28b as already noted and the control region 32 continues to act as the loss region. On the other hand, the carriers are not removed from the saturable absorption region 31 and there appears a decrease in the absorption coefficient in the region 31 responsive to the injection of the set optical pulse. In other words, the loss in the saturable absorption region 31 is reduced as a result of injection of the set optical pulse. Thus, when the gain in the gain region 30 has exceeded the overall loss caused in the saturable absorption region 31 and in the control region 32 as well as at the mirrors E₁ and E₂, the laser diode starts to oscillate and the output power is increased rapidly until it reaches the optical high level state. Note that loss in the saturable absorption region 31 is rapidly decreased once there is established a sufficient optical radiation in the active layer 23. Thus, the optical radiation increases its power steeply when the laser oscillation is started. As the longitudinal length L₂ of the control region 32 is set small as already described, the oscillation of the laser diode occurs easily and efficiently although the control region 32 continues to act as the loss region also in the oscillating state. The laser diode continues its high optical state even when the set optical pulse is removed. Next, transition of the laser diode from the turned-on state to the turned-off state will be described. When it is desired to turn off the laser diode, the loss in the control region 32 is increased such that the overall loss of the laser diode exceeds the gain in the gain region 30. For this purpose, a reset voltage pulse of about - 0.25 volts is applied to the electrode 28b through an input terminal 42 and a blocking capacitor 43. Responsive thereto, the carriers in the control region 32, which is already small in the turned-on state, is removed further therefrom and the loss in the control region 32 is increased. FIG.6 shows a relation between the gain or absorption coefficient of the material forming the control region and the carrier density therein for the optical bistable laser diode of the prior art and the present invention. In the drawing, a curve C₁ represents the turned-on state of the prior art optical bistable laser diode shown in FIG.1. As can be seen in the drawing, there is a peak of gain at a wavelength of laser oscillation when there exists a large carrier density n₁ which may be above 10⁻¹⁸ cm⁻³ in correspondence to the drive current I₂. On the other hand, a curve C₂ represents the state in which the laser diode is turned off and the carrier density is reduced to n₂ responsive to the reset current pulse. In this state, the again of the control region 17 of the device of FIG.1 is reduced to substantially zero. As already noted, the reset current pulse removes the carrier more or less thoroughly from the control region 17 so as to obtain quick a turn-off. Associated therewith, there is caused the problem of long dead time. A curve C₃, on the other hand, represents the turned-on state of the laser diode of the present invention wherein there exists the carrier in the control region 32 with a density n₃ in correspondence to the voltage V applied to the electrode 28b. Note that the carrier density n₃ is significantly smaller than the carrier density n₂ and the control region 32 of the device of FIG.4 operates as the loss region even in the turned-on state of the laser diode. Further, a curve C₄ represents a state in which the foregoing reset voltage pulse is applied to the electrode 28b so as to remove the carriers from the control region 32. Responsive thereto, the the carrier density is reduced from n₃ to n₄. This change of the carrier density from the level n₃ to n₄ is far smaller than the change from the level n₁ to n₂ as the initial current density n₃ is already set to a very small level. The level of the point n₄ may be 10⁺¹⁷ cm⁻³, for example. The foregoing relation is summarized as n₁ > n₂ > n₃ > n₄, and n₁ - n₂ >> n₃ - n₄. Referring to FIG.6 again, it will be noted that the change of absorption coefficient caused in the case of the present invention is much larger than the corresponding change of gain in the prior art device. This change of the absorption coefficient is caused by the change of the carrier density from n₃ to n₄ of which magnitude is smaller by a factor of ten or more than that of the carrier density change from n₁ to n₂. In other words, there is induced a large loss in the control region 32 responsive to the minute decrease of the carrier density from n₃ to n₄ and the oscillation of the laser diode is immediately stopped. As the change of the carrier density associated with the reset voltage pulse is minute in the case of the present invention, the recovery of the carrier density n₃ after the removal of the reset voltage pulse is achieved easily and the laser diode can be turned on again immediately after the turn-off by the optical set pulse having a small optical power. FIGS.7(A) and (B) show a hysteresis curve pertinent to the operational characteristic of the laser diode of the present invention in which FIG.7(A) shows the change of the current supplied to the laser diode under a condition that there is no external optical injection and a constant voltage of 0.55 volts is applied to the second electrode 28b as the voltage V . Under this situation, the hysteresis loop is obtained by increasing and decreasing the current I supplied to the first electrode 28a about a current level of 45.5 mA which corresponds to the center of the hysteresis curve. In this case, the width of the hysteresis for the current I is about 3 mA. FIG.7(B) shows a case wherein a constant current of 45.5 mA is supplied to the gain region through the first electrode 28a as the current I and the voltage V applied to the second electrode 28b is increased or decreased about a voltage of 0.55 volts. Similarly to the case of FIG.7(A), there is no external injection to the laser diode. As is clearly seen in the drawing, the laser diode is turned on responsive to the increase of the voltage V above the voltage of about 0.59 volts and is turned off responsive to the decrease of the voltage V to below about 0.51 volts. FIG.8 shows the result of experiment undertaken to evaluate the response of the laser diode of FIG.4 by providing the set optical pulses and reset voltage pulses alternately. In the illustrated experiment, the laser diode was driven by setting the current I to 45.5 mA in correspondence to the center of the hysteresis and by setting the voltage V to 0.55 volts. Further, the laser diode was reset or turned off each time when a new operational cycle is started by applying a negative voltage pulse having an amplitude of - 0.25 volts as the reset voltage pulse. In other words, the voltage V at the second electrode 28b is momentarily reduced to 0.30 volts in correspondence to the reset voltage pulse. The reset voltage pulse used for the experiment has a pulse width of 200 picoseconds and a frequency of 500 MHz. Further, in order to turn the laser diode on, an optical pulse having a wavelength of 1.307 µm and optical power of 390 µW was applied as the set optical pulse with a pulse width of 200 picoseconds. The set optical pulse was provided in correspondence to a series of binary digit "1", "1", "0" and "1" in the illustrated example. As can be seen in the drawing, the laser diode can be operated properly even when the time interval between the reset voltage pulse and the optical set pulse was reduced to below 1 nanosecond. In the prior art device, this time interval cannot be reduced below several nanoseconds because of the excessive increase of optical power of the optical set pulse. FIGS.9(A) and (B) compare the time interval needed for the laser diode to become operational after the voltage pulse or current pulse for resetting the laser diode is removed. In FIG.9(A) corresponding to the laser diode of the present invention, it will be apparent that the change of the carrier density responsive to the reset voltage pulse is very small and the laser diode can be turned on after the previous turn-off by injecting the optical set pulse having a very small optical power. Even when the time interval between the reset voltage pulse and the set optical pulse or dead time D is reduced, the optical power needed for the set optical pulse increases only slightly. In FIG.9(B) corresponding to the prior art laser diode, on the other hand, there occurs a significant change of carrier density responsive to the large reset current pulse and it needs a large optical power for the set optical pulse in order to turn on the laser diode subsequently to the previous turn-off. Note that the optical power needed to turn on the laser diode increases steeply with decrease of the dead time D . Responsive to the decrease of the dead time D , the waiting time of the device which is an interval T between the trailing edge of the reset voltage pulse or reset current pulse and the leading edge of the next set optical pulse is decreased significantly by operating the laser diode according to the control method disclosed heretofore. FIG.10 shows the relation between the waiting time T and the minimum optical power needed for the set optical pulse to turn the laser diode on. As represented by the solid circles in the drawing, the power needed for the set optical pulse increases sharply in the case of the prior art laser diode when the time interval T is decreased below a few nanoseconds. Clearly, the decrease of the time interval T below about two or three nanoseconds is impossible in the prior art device because of the excessive increase of the optical power needed for the set optical pulse. On the contrary, the relation for the present invention represented in the drawing by open circles clearly shows that the time interval T can be reduced to zero even though there is a slight increase in the optical power for the optical set pulse. Thus, the laser diode of the present invention controlled as described heretofore can respond almost immediately whenever the reset voltage pulse is removed and the speed of operation is improved significantly. Such an improvement enables the use of high clock frequency when the laser diode is used as the optical logic device. Finally, the steps for manufacturing the laser diode of FIG.4 will be described briefly with reference to FIGS.11(A) - (C). First, the clad layer 22, active layer 23, the second clad layer 24 and the contact layer 25 are grown on the substrate 21 by liquid phase epitaxy (LPE) with a thickness of 2.0 µm for the clad layer 22, 0.15 µm for the active layer 23, 1.5 µm for the clad layer 24 and 0.3 µm for the contact layer 25 to form a structure shown in FIG.11(A). Next, the insulator film 33 is provided on the contact layer 25 with a thickness of 3000 Å (300 nm). This film 33 is then patterned photolithographically such that the film 33 extends along the center of the structure of FIG.11(A) from the end E₁ to the other end E₂ with a width of 4.0µm. Next, using the film 33 as the mask, the structure of FIG.11(A) is subjected to etching until the etching reaches a top part of the clad layer 22. As a result, a structure shown in FIG.11(B) is obtained wherein the layers 23 - 25 as well as the top part of the clad layer 22 are removed except for a central part 37 protected by the insulator film 33. During this procedure, the width of the central part 37 is reduced to 1.5 µm by etching which acts also laterally to some extent. Next, the buried layer 26 having high resistivity is formed at both sides of the central part 37 by growing indium phosphide doped with iron by metal-organic chemical vapor deposition (MOCVD) for a thickness of 2.5 µm to form a structure shown in FIG.11(C). Next, the insulator film 33 is removed and the insulator film 27 is deposited thereon with a uniform thickness of 3000 Å (300 nm). The insulator film 27 is then subjected to selective etching such that there is formed the opening 34 extending along the central part 37 from the end E₁ to the other end E₂. As a result of the opening 34, the contact layer 25 is exposed. Next, the metal layer 35a which in turn comprises a stacking of a titanium layer having a thickness of 1000 Å (100 nm) and a platinum layer also having a thickness of 1000 Å (100 nm) is deposited on the structure thus obtained by electron beam deposition. The structure is then heat treated at 430°C for 30 minutes. Further, a 2 µm-thick gold layer is plated selectively on the metal layer 35b except for the region corresponding to the saturable absorption region 31. Thus, the metal layer 35b is provided only in correspondence to the gain region 30 and the control region 32. Further, the metal layer 35a and the insulator film 37 corresponding to the saturable absorption region 31 are selectively removed by reactive ion etching (RIE) using argon gas for platinum and carbon tetrafluoride (CF₄) for titanium to expose the top of the buried layer 26 as well as the contact layer 25. This contact layer 25 in the saturable absorption region 31 is further removed by selective etching. Further, the thickness of the substrate 21 is adjusted by removing the bottom of the substrate such that the substrate 21 has a thickness of 100 µm, and the metal layer 36a which in turn comprises a gold-germanium alloy layer with a thickness of 2000 Å (200 nm) and a gold layer with a thickness of 300 Å (30 nm) is deposited on the bottom of the substrate 21 thus prepared. The structure is then heat treated at 380°C for one minute and is further covered by another gold layer corresponding to the metal layer 36b with a thickness of 2 µm by plating. Thus the metal layer 36b is deposited on the metal layer 36a and the structure shown in FIG.11(D) corresponding to the device shown in FIG.4 is completed.

Claims: A method of controlling an optical bistable laser diode, of the type comprising : a first semiconductor clad layer (22), a second semiconductor clad layer (24), an active layer (23) provided so as to be sandwiched between said first and second semiconductor clad layers and defined by a first end and an opposing second end, said first end and said second end opposing each other in a longitudinal direction of said laser diode, reflection means (E₁, E₂) provided on said first and second ends so as to form a resonator in the active layer, first and second electrodes (28a, 28b) provided on said second semiconductor clad layer substantially in alignment in a direction connecting the first end and the second end, said first electrode (28a) having a length (L₁) in said longitudinal direction and defining a gain region in said active layer in correspondence to said first electrode by injecting current into said active layer, said gain region amplifying an optical beam therein as it reflects back and forth at said reflection means, and a gap region (L₃) defined between the first and second electrodes, said gap region defining a saturable absorption region (31) in a part of said active layer located underneath, wherein said first electrode, said second electrode and said gap region respectively have lengths (L₁), (L₂) and (L₃) measured along said direction satisfying a relation (L₂ + L₃/2) / (L₁ + L₂ + L₃) ≦ 0.3 characterized in that it involves applying via said second electrode (28b) a voltage to said active layer such that a loss region is formed in said active layer in correspondence with said second electrode, said loss region causing an absorption of an optical beam passing therethrough in response to an application of said voltage to said second electrode. The method as claimed in claim 1 wherein said lengths L1, L2 and L3 substantially satisfy a relation (L2 + L3/2) / (L1 + L2 + L3) ≅ 0.1. The method as claimed in claim 1 wherein said active semiconductor layer (23) is laterally bounded by a pair of other semi-insulating semiconductor layers (26) for lateral confinement of an optical radiation formed in the active semiconductor layer. The method as claimed in claim 1 wherein the laser diode further comprises a contact layer (25) of semiconductor material doped to the second conduction type between said second semiconductor clad layer (24) and said first electrode (28a) and between said second semiconductor clad layer and said second electrode (28b) for improving electrical contact. An optical system comprising : an optical bistable diode having a first semiconductor clad layer (22), a second semiconductor clad layer (24), an active layer (23) provided so as to be sandwiched between said first and second semiconductor clad layers and defined by a first end and an opposing second end, reflection means (E₁, E₂) provided on said first and second ends so as to form a resonator in the active semiconductor layer, first and second electrodes (28a, 28b) provided on said second semiconductor clad layer substantially in alignment in a direction connecting the first end and the second end, a gap region (L₃) defined between the first and second electrodes, said gap region defining a saturable absorption region (31) in a part of said active layer coated underneath, and first biasing means (44) for injecting a current to said first electrode with a level capable of establishing a laser oscillation, characterized in that the system further comprises : second biasing means (40, 40a, 41) for applying a biasing voltage to said second electrode with a level such that a part (32) of the active layer underneath said second electrode acts as an optical loss region by providing an optical loss to an optical radiation passing therethrough, and control means (42, 43) for controlling the bias voltage such that the optical loss in said optical loss region is increased and the laser oscillation is stopped. An optical bistable laser apparatus as claimed in claim 5 characterized in that said first electrode (28a), said second electrode (28b) and said gap region (L₃) respectively have lengths L1, L2 and L3 measured in said direction satisfying a relation (L2 + L3/2) / (L1 + L2 + L3) ≦ 0.3. An optical bistable laser apparatus as claimed in claim 5, characterized in that said second biasing means (40, 40a, 41) removes carriers from said optical loss region (32) to the ground. An optical bistable laser apparatus as claimed in claim 5 characterized in that said second biasing means comprises a constant voltage source (40) and a choke coil (40a) connected in series. A method of controlling an optical bistable laser diode having an active layer (23) sandwiched between a pair of clad layers (22, 24) with a gain region (30) defined in the active layer for producing an optical radiation, a saturable absorption region (31) defined in the active layer adjacent to the gain region for transmitting the optical radiation produced in the gain region with a variable loss which changes responsive to a power of the optical radiation, and a control region (32) defined in the active layer adjacent to the saturable absorption region for receiving the optical radiation from the saturable absorption region and transmitting it with a controlled gain, characterized by the steps of: injecting a predetermined drive current to the gain region with a level capable of causing a bistable operation of the laser diode to establish said optical radiation in the gain region, applying a predetermined finite voltage to the control region such that the control region is induced with an optical loss, and switching the laser diode to a turn-on state by injecting an optical pulse to the active layer. A method as claimed in claim 9, characterized in that said finite voltage is determined in correspondence with a diffusion potential established between a material forming said clad layers (22, 24) and a material forming the active layer (23). A method as claimed in claim 9, characterized in that said finite voltage is determined such that the current flowing through the control region (32) is 100 µA or less. A method as claimed in claim 9, characterized in that said finite voltage is determined equal to or lower than a characteristic voltage level (A) above which a current flowing through the control region is increased steeply and below which substantially no current flows through the control region, and in that said method further comprises the steps of switching the lower diode to a turn-off state by applying a voltage lower than said predetermined finite voltage to control the region in the form of a reset voltage pulse.

Inventors: ODAGAWA TETSUFUMI, SANADA TATSUYUKI, ODAGAWA, TETSUFUMI, SANADA, TATSUYUKI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: G02F3
IPC codes: H01S5, G02F3