JPH10135565A - Mode-locked semiconductor laser and method of changing repetition rate frequency - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to change the repetition rate of a pulse train easily by providing a passive wave guide region with a double heterostructure which changes its equivalent refractive index according to the applied reverse bias voltage and thereby changes the repetition rate of mode-locked light. SOLUTION: In a passive wave guide region 26, an n-InP clad layer 10, a passive wave guide (i-InGaAsP layer) 12, and a p-InP clad layer 16 constitute a double heterostructure. When reverse bias voltage is applied to the double hetero-structure, a refractive index of the i-InGaAsP layer changes due to the Pockels effect and, as a result, an equivalent refractive index of the passive wave guide region 26 changes. The equivalent refractive index of the passive wave guide 26 increases with the increase of applied voltage and therefore a cycle frequency can be changed continuously. By this method, the repetition rate can be easily controlled just by changing the magnitude of the reverse bias voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mode-locked semiconductor laser which is used as a light source for optical measurement and optical communication and generates an ultrashort optical pulse train.

[0002]

2. Description of the Related Art Conventionally, there is a mode locking method as one of laser oscillation methods for generating an ultrashort optical pulse train. The mode-locking method is a method of obtaining an optical pulse train by synchronizing the phases of laser beams that oscillate in multiple longitudinal modes.
In order to perform mode locking by synchronizing the phase of the laser light oscillating in each longitudinal mode, the gain or loss of the light propagating in the resonator is determined by a frequency (hereinafter, referred to as a circulating frequency) that matches the longitudinal mode interval. .) Or at an integral multiple of this frequency.

[0003] The mode locking method is roughly classified into an active mode locking method and a passive mode locking method. The active mode locking method is a method of externally modulating the gain or loss of light propagating in a laser resonator at a circulating frequency or a frequency that is an integral multiple of this. Conventionally, as a semiconductor laser using this active mode-locking method, reference 1 [IEEE Photon.Tech.
nol. Lett. 4, 215 (1992) ".

The passive mode-locking method is a method in which a saturable absorber is inserted into a resonator to generate an optical pulse train having a frequency coincident with the circulating frequency without requiring external modulation. The saturable absorber has the property of being transparent for high-intensity light and acting as an absorber for low-intensity light, and the propagation loss in the resonator modulates spontaneously. And cause mode synchronization. This passive mode-locking method has the advantage that the generated mode-locked light has a small optical pulse width and an optical pulse of the conversion limit is easily obtained. For example, in the case of a semiconductor laser using the passive mode-locking method disclosed in Reference 2, "IEEE Photon.Technol. Lett. 5,1362 (1993)", the repetition frequency is 40.8 GHz and the pulse width is 3.5 ps. The conversion limit pulse has been obtained.

As described above, an optical pulse having a pulse width on the order of several picoseconds can be obtained by the mode locking method.

[0006]

However, in the case of a mode-locked laser, the repetition frequency of the generated optical pulse train (hereinafter sometimes referred to as mode-locked light or emitted light) is determined by the circulating frequency. Changing the repetition frequency required changing the length of the resonator. For this reason, conventionally, in order to change the repetition frequency of the light emitted from the mode-locked semiconductor laser, a configuration provided with an external resonator (for example, see Reference 3, “abstracts of FST'95, p. 51, 199”).
5 ". ), And the optical axis must be adjusted each time the length of the external resonator (resonator length) is changed, which is very troublesome.

Therefore, there has been a demand for a mode-locked semiconductor laser capable of easily changing the repetition frequency of a generated optical pulse train and a method of changing the repetition frequency.

[0008]

A mode-locked semiconductor laser according to the present invention has a structure in which a gain region and a passive waveguide region are integrated in a single semiconductor substrate, and generates mode-locked light. In the mode-locked semiconductor laser, the passive waveguide region has a double hetero structure in which a repetition frequency of the mode-locked light changes by changing an equivalent refractive index according to a reverse bias voltage.

As described above, by providing a passive waveguide region in which the equivalent refractive index changes in accordance with the applied reverse bias voltage as a part of the resonator, the effective length of the resonator according to the reverse bias voltage is provided. (Optical distance), and thus the repetition frequency of the generated mode-locked light can be easily changed. Here, the equivalent refractive index is a numerical value obtained by standardizing and expressing the relationship between the propagation constant and the wave number of light propagating in the optical waveguide (also referred to as a normalized propagation constant). Here, it is an amount that determines the group velocity of light propagating in the passive waveguide region, and is an amount that is determined by the refractive indices of both materials of the core and the cladding layer constituting the passive waveguide region and the waveguide structure.

Further, according to the method of varying the repetition frequency of the present invention, a mode-locked semiconductor which has a structure in which a gain region and a passive waveguide region are integrated in a single semiconductor substrate and generates mode-locked light is provided. In changing the repetition frequency of the mode-locked light of the laser, the repetition frequency of the mode-locked light is changed by applying a reverse bias voltage to the passive waveguide region and changing the equivalent refractive index of the passive waveguide region. It is characterized by the following.

As described above, by applying a reverse bias voltage to the passive waveguide region forming the resonator, the equivalent refractive index of the passive waveguide region changes, and thus the length of the resonator changes effectively. However, the repetition frequency of the generated mode-locked light can be easily changed.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. It should be noted that the drawings are schematic to the extent that the size, shape, and arrangement of the present invention can be understood, and the numerical conditions and the like described below are merely examples. It is not limited in any way.

FIG. 1 is a sectional view showing the structure of the embodiment. This configuration example is an example of a passive mode-locked semiconductor laser in which a saturable absorption region is integrated in a single semiconductor substrate in addition to a gain region and a passive waveguide region.
In this configuration example, an n-InP cladding layer 10 is provided as a semiconductor substrate, and a passive waveguide 12 and an active waveguide 14 are provided on the upper side of the n-InP cladding layer 10 in a light guiding direction (arrow a in the drawing). In the direction indicated by.). A p-InP cladding layer 16 is provided above the passive waveguide 12 and the active waveguide 14, and a block layer (not shown) is provided on both sides of the passive waveguide 12 and the active waveguide 14. ing. Then, the passive waveguide 12 and the active waveguide 14 are formed by this block layer and n
-InP cladding layer 10 and p-InP cladding layer 16
To form a resonator 38. still,
When the film thickness of the n-InP cladding layer 10 is 150 μm and p-I
The thickness of the nP cladding layer 16 is 2 μm.

An n-side electrode 18 is provided on the entire lower surface of the n-InP cladding layer 10, and three p-side electrodes 20, 22 and 2 are provided on the upper surface of the p-InP cladding layer 16.
4 are provided. The p-side electrode 20 is connected to the p-I so that a voltage can be applied to the passive waveguide 12 between the p-side electrode 20 and the n-side electrode 18.
It is provided at a position on the upper surface of the nP cladding layer 16. p
The side electrodes 22 and 24 are provided at positions on the upper surface of the p-InP clad layer 16 so that current can be injected and voltage can be applied to the active waveguide 14 between the n-side electrode 18.

Here, the p-side electrode 20 and the n-side electrode 18
A reverse bias voltage source 33 for applying a reverse bias voltage is connected between them, and a region to which the voltage is applied is called a passive waveguide region 26. Also, the p-side electrode 22 and n
A forward bias current source 34 is connected between the side electrode 18 so that a current can be injected in the forward direction, and a region into which the current is injected is a gain region 28. The gain region 28 is a region where a current is injected to function as a gain medium and generate light and amplify the generated light necessary for laser oscillation.

A reverse bias voltage source 36 is connected between the p-side electrode 24 and the n-side electrode 18 so that a reverse bias voltage is applied. The region to which the reverse bias voltage is applied is the saturable absorption region 30. When a reverse bias voltage is applied to the saturable absorption region 30, the resonator 3
This is a region that functions as an optical switch for light guided inside 0.

In this embodiment, the n-side electrode 18, the p-side electrodes 20, 22 and 24 are formed as ohmic electrodes. The p-side electrodes 20, 22 and 24 transfer Au to p
-Obtained by vapor deposition on the entire upper surface of the InP cladding layer 16 and forming a separation portion by chemical etching.

The passive waveguide 12 is composed of an i-InGaAsP layer having a band gap wavelength of 1.2 μm.
Together with the n-InP cladding layer 10 and the p-InP cladding layer 16 provided above and below, a passive waveguide region 26 having a double hetero structure is formed.

The active waveguide 14 has a semiconductor quantum well structure 4 having a band gap wavelength of about 1.56 μm between the InGaAsP guide layer 40 and the InGaAsP guide layer 42.
4. The semiconductor quantum well structure 44 has a well of InGaAs (bandgap wavelength 1.67 μm) having a thickness of 7 nm and a barrier having a thickness of 14 n.
m InGaAsP (band gap wavelength 1.2 μm)
By alternately laminating these three layers at a time,
It is composed of a total of six layers. The semiconductor quantum well structure 4
InGaAsP guide layer 40 provided on the upper side of
Is 120 nm, and the thickness of the InGaAsP guide layer 42 provided below the semiconductor quantum well structure 44 is 60 nm.

The length (resonator length) of the resonator 38 in this embodiment along the waveguide direction is 2.45 mm.
The length of each of the regions constituting the resonator 38 along the waveguide direction is such that the length of the passive waveguide region 26 is 1460 μm, the length of the gain region 28 is 940 μm, and the saturable absorption region 3 is
The length of 0 is configured as 50 μm.

As described above, the passive waveguide 12 constituting the mode-locked semiconductor laser of this embodiment is formed of a material having a band gap wavelength sufficiently shorter than the band gap wavelength of the active waveguide 14. And also
A material that propagates light generated in the active waveguide 14 with low propagation loss is used. Further, as described later, the material of the passive waveguide 12 is also a material whose refractive index changes according to application of a reverse bias.

Each of the above-mentioned regions (passive waveguide region 2
6, electrical isolation is maintained between the gain region 28 and the saturable absorption region 30), and the electrical interference effect is sufficiently suppressed.

Next, the operation of the mode-locked semiconductor laser of this embodiment will be described. First, passive mode synchronization will be described. The passive mode locking is generated by the optical switch function of the saturable absorption region 30 and the optical amplification function of the gain region 28 functioning together. The light absorption coefficient of the saturable absorption region 30 changes according to the intensity of the incident light, and exhibits such a characteristic that the absorption coefficient decreases as the light intensity increases. Therefore, the saturable absorption region 30 is a spontaneous switch that selectively transmits only light having a relatively high light intensity of the input optical signal and does not transmit light having a relatively low light intensity. work. Since this switching action repeatedly acts on the light guided in the resonator 38, the optical signal is sharpened (pulsed waveform) and passive mode locking occurs. In the operation process described above, the gain region 28 has a function of compensating for a propagation loss in the resonator 38 and providing a gain for realizing laser oscillation.

In this passive mode-locking operation, the passive waveguide region 26 does not essentially contribute. The passive waveguide region 26 has a role of determining the resonator length of the resonator 38 and determining the repetition frequency of the generated optical pulse train. Next, the relationship between the repetition frequency and the resonator length will be described.

The repetition frequency f _rt of the generated optical pulse train (mode-locked light) is determined by the effective resonator length, and substantially coincides with the following equation (1).

F _rt = c / {2 ( _nal L _al + n _pc L _pc )} (1) where c is the speed of light in a vacuum, and n _al is the gain region 28 and the saturable absorption region 30. And the equivalent refractive index of n _pc
Represents the equivalent refractive index of the passive waveguide region 26. Also, L _al
Is the length of the gain region 28 and the saturable absorption region 30 in the direction along the optical waveguide direction a, and L _pc is the passive waveguide region 26
Is the length along the optical waveguide direction a. The sum of L _al and L _pc is the actual length of the resonator 38, and the length (n _al L _al
+ N _pc L _pc ) is the effective length of the resonator in consideration of the refractive index of the medium.

Thus, the substantially passive waveguide region 26
Of be changed to n _pc L _pc representing the length, results in changing the substantial length of the resonator 38, the repetition frequency f _rt
Is obtained. According to such a method, it is understood that the repetition frequency can be changed only by electrical means, and no complicated means is required.

In the mode-locked semiconductor laser of this embodiment, the repetition frequency of the mode-locked light (emitted light) is changed in the passive waveguide region 26 by changing the equivalent refractive index n _pc according to the applied reverse bias voltage. It has a double heterostructure in which f _rt changes. As described above, the passive waveguide region 26 forms a double hetero structure with the n-InP cladding layer 10, the passive waveguide (i-InGaAsP layer) 12, and the p-InP cladding layer 16. When a reverse bias voltage is applied to this double hetero structure, the refractive index of the i-InGaAsP layer changes due to the Pockels effect, and as a result, the equivalent refractive index n _pc of the passive waveguide region 26 changes.

As described above, the equivalent refractive index n _pc of the passive waveguide region 26 increases as the applied voltage increases, and as a result, the repetition frequency f _rt can be continuously changed (repetition frequency). _frt decreases as the applied voltage increases). Therefore, the mode-locked semiconductor laser of this embodiment can easily adjust the repetition frequency only by changing the magnitude of the reverse bias voltage, and only the electric means can be used to change the repetition frequency. Since it is used, it can be used after being integrated with another electric circuit.

Next, the results of actual measurement of the characteristics of this embodiment will be described. First, when a current was uniformly injected into the saturable absorption region 30 and the gain region 28 of the fabricated device (the mode-locked semiconductor laser of this embodiment), the threshold value of the device was about 20 mA. . When a reverse bias voltage of about 0.3 V was applied to the saturable absorption region 30 and a current of about 100 mA was injected into the gain region 28, it was confirmed that an optical pulse train was generated.

FIG. 2 is a graph showing the measurement results of the SHG autocorrelation waveform of this embodiment. The horizontal axis shows the delay time for each 8.47 ps, and the vertical axis shows the SHG intensity as an arbitrary intensity (au). As an envelope of the measured pulse waveform, sech (generally, sechx
^{= 2 / (e x + e} -x) results were analyzed assuming a square-shaped curve of the given), the pulse width was estimated 2.7Ps. In addition, as a result of examination with a microwave spectrum analyzer, the pulse repetition frequency was 17.67264 GHz.
z, which is in good agreement with the circulating frequency when the resonator length is 2.45 mm. Therefore, it was confirmed that an optical pulse train was generated by passive mode locking.

FIG. 3 is a graph showing the measured results of the relationship between the reverse bias voltage applied to the passive waveguide region 26 of this embodiment and the pulse repetition frequency of the generated light.
In the figure, the abscissa indicates the reverse bias voltage (the forward bias direction is positive), and the ordinate indicates the pulse repetition frequency. The measurement is performed by setting the reverse bias voltage to-
The test was performed by changing the voltage from 2.8V to 0V every 0.4V. The measurement data of the repetition frequency with respect to the reverse bias voltage is indicated on the graph by a black circle. As is clear from the figure,
When the reverse bias voltage is increased from -2.8V to 0V, the repetition frequency of the mode-locked light becomes about 17.68 GHz.
From about 17.6726 GHz. In this way, a change amount (tuning bandwidth) of the repetition frequency of about 7.4 MHz was obtained. At this time, there was almost no change in the pulse width.

According to the mode-locked semiconductor laser of this embodiment, as described above, the equivalent refractive index n _pc of the passive waveguide region 26 is changed by applying the reverse bias voltage to generate the Pockels effect. I decided that
There is an advantage that the function of the saturable absorption region 30 does not deteriorate due to the propagation loss of the passive waveguide 12 when tuning the repetition frequency.

That is, it is possible to change the equivalent refractive index n _pc of the passive waveguide region 26 by generating a plasma effect by injecting a forward current into the passive waveguide region 26. In this case, In this case, there is a disadvantage that mode locking cannot be performed because the propagation loss of the passive waveguide 12 increases with an increase in injection current. This is because, when the amount of injected current is large, the absorption of light by free carriers in the passive waveguide 12 increases as the carrier density increases, and the light intensity in the resonator 38 attenuates accordingly. This is because the light intensity required to make the saturable absorption region 30 transparent cannot be obtained, and therefore, the saturable absorption region 30 cannot fulfill the optical switch function.

The change in the propagation loss in the passive waveguide 12 can be known by measuring the change in the spectral purity of the emitted light. Here, the spectral purity indicates the variation of the oscillation frequency with respect to the set frequency. That is, the higher the light intensity of the oscillation component (background component) having a frequency shifted from the set frequency, the lower the spectral purity. When the propagation loss increases, the Q value decreases, and the spectral purity deteriorates. That is, a decrease in the value of the spectral purity indicates that the propagation loss increases.

FIG. 4A is a graph showing the relationship between the current injected into the passive waveguide 12 and the spectral purity. As can be seen from the figure, the injection of the forward current causes the equivalent refractive index n _pc
Is changed, the spectral purity decreases as the current injection amount increases (therefore, the propagation loss increases). This is considered to be because the optical switch operation of the saturable absorption region 30 is deteriorated with the decrease in the light intensity in the resonator 38, and the coupling between the longitudinal modes of the laser is weakened. This suggests that the mode synchronization function is deteriorated.

On the other hand, FIG. 4B is a graph showing the relationship between the value of the reverse bias voltage applied to the passive waveguide 12 and the spectral purity in the mode-locked semiconductor laser according to this embodiment. As can be seen from this figure, the spectral purity does not change even if the value of the reverse bias voltage is changed. This indicates that the Q value of the resonator 38 does not change, so that even if the value of the reverse bias voltage is changed, the propagation loss of the passive waveguide 12 does not change.

As described above, when the equivalent refractive index n _pc is changed by applying the reverse bias voltage, the propagation loss of the passive waveguide 12 does not substantially change even if the amount of the applied reverse bias voltage is changed. Therefore, the repetition frequency can be changed without deteriorating the pulse generation characteristics.

The propagation loss when a reverse bias voltage is applied may be caused by a shift in the band edge due to the Franz-Keldysh effect. The mode-locked semiconductor laser according to this embodiment is considered. In the case where the band gap of the passive waveguide 12 is on the wavelength side sufficiently shorter than the laser oscillation wavelength, the influence of the band edge shift due to the Franz-Keldysh effect can be ignored.

As is apparent from the above description, according to the mode-locked semiconductor laser and the method of varying the repetition frequency of this embodiment, the repetition frequency is changed by changing the value of the reverse bias voltage applied to the passive waveguide. It is possible to change.

Since the repetition frequency is controlled by the reverse bias voltage applied to the passive waveguide, the pulse generation characteristics do not change with the change in the frequency.

[0042]

As described in detail above, according to the mode-locked semiconductor laser of the present invention, the passive waveguide region whose equivalent refractive index changes according to the reverse bias voltage is provided as a part of the resonator. The repetition frequency of the optical pulse train output from the mode-locked semiconductor laser can be changed according to the applied voltage. Since the repetition frequency can be changed only by the electrical means of changing the applied voltage, a complicated procedure such as adjustment of the optical axis is not required. Further, since only electric means is used as means for changing the repetition frequency, it is also possible to use it by integrating it with another electric circuit. Further, since the repetition frequency is controlled by the reverse bias voltage, the pulse generation characteristics do not change with the change of the frequency.

According to the method of varying the repetition frequency of the present invention, the repetition frequency of the optical pulse train can be changed by changing the equivalent refractive index of the passive waveguide according to the applied voltage. The change can be easily performed without changing the pulse generation characteristics.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a configuration of an embodiment.

FIG. 2 is a graph showing an SHG autocorrelation waveform according to the embodiment.

FIG. 3 is a graph showing a relationship between a reverse bias voltage and a repetition frequency according to the embodiment.

4A is a graph showing a relationship between a current injected into a passive waveguide and spectral purity, and FIG. 4B is a graph showing a relationship between a reverse bias voltage applied to the passive waveguide and spectral purity. It is.

[Explanation of symbols]

 10: n-InP cladding layer 12: passive waveguide 14: active waveguide 16: p-InP cladding layer 18: n-side electrode 20, 22, 24: p-side electrode 26: passive waveguide region 28: gain region 30: Saturable absorption region 33, 36: reverse bias voltage source 34: forward bias current source 38: resonator 40, 42: InGaAsP guide layer 44: semiconductor quantum well structure

Claims (6)

[Claims] 1. A mode-locked semiconductor laser having a structure in which a gain region and a passive waveguide region are integrated in a single semiconductor substrate. A mode-locked semiconductor laser comprising a double heterostructure in which a repetition frequency of the mode-locked light changes by changing an equivalent refractive index according to a voltage. 2. The double heterostructure of the passive waveguide region, wherein an equivalent refractive index is changed by a Pockels effect caused by applying a reverse bias. Mode-locked semiconductor laser. 3. The double heterostructure of the passive waveguide region comprises a p-InP layer, an i-InGaAsP layer, and an n-type InGaAsP layer.
3. The mode-locked semiconductor laser according to claim 1, wherein the mode-locked semiconductor laser is composed of an InP layer. 4. A mode-locked semiconductor laser for generating a mode-locked light having a structure in which a gain region and a passive waveguide region are integrated in a single semiconductor substrate, wherein the repetition frequency of the mode-locked light is changed. A method of changing the repetition frequency of the mode-locked light by applying a reverse bias voltage to the passive waveguide region to change an equivalent refractive index of the passive waveguide region. 5. The method of claim 4, wherein the equivalent refractive index is changed by a Pockels effect caused by applying a reverse bias voltage to the double hetero structure in the passive waveguide region. . 6. Changing the equivalent refractive index by applying a reverse bias voltage to the passive waveguide region formed of the double hetero structure of a p-InP layer, an i-InGaAsP layer, and an n-InP layer. 6. The method of claim 4, wherein the repetition frequency is varied.